User manual: small updates

A section on multicylinder dives has been added.
Just to warm LongJohnSilver's heart, a short section on pSCR has been added.
Please feel free to add to the pSCR section, LongJohn.
Replaced the sidemount figure with obsolete mean depth line.

Fixes #827
Fixes #828

Signed-off-by: willem ferguson <willemferguson@zoology.up.ac.za>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

Documentation/user-manual.txt

@@ -824,6 +824,7 @@ _Subsurface_ and the information on
 cylinders and gases (entered here) determines the behaviour of the dive profile
 (top right-hand panel).
 
+[[cylinder_definitions]]
 *Cylinders*: The cylinder information is entered through a dialogue that looks
 like this:
 
@@ -1635,12 +1636,46 @@ If, later, the external drive with the photos is connected again, the photos can
 
 === Logging special types of dives
 
+[[S_MulticylinderDives]]
+==== Multicylinder dives
+
+_Subsurface_ easily handles dives involving more than one cylinder. Multicylinder diving usually happens
+(a) if a diver does not have enough gas for the complete dive in a single cylinder; (b) if the diver
+needs more than one gas mixture because of the depth or the decompression needs of the dive. For this reason
+multicylinder dives are often used by technical divers who dive deep or long. As far
+as _Subsurface_ is concerned, there are only two types of information that need to be provided:
+
+- *Describe the cylinders used during the dive* This is performed in the *Equipment tab* of
+  the *Dive Info* panel, as xref:cylinder_definitions[described above]. Enter the cylinders one by one,
+  specifying the characteristics of the cylinder and the gas composition within each cylinder.
+
+- *Record the times at which switches from one cylinder to another was done:* This is information
+  provided by some dive computers (provided the diver indicated these changes to the dive computer
+  by pressing specific buttons). If the dive computer does not provide the information, the diver has to
+  record these changes using a different method, e.g. writing it on a slate.
+
+- *Record the cylinder changes on the dive profile*: If the latter option
+  was followed, the diver needs to indicate the gas change event by right-clicking at the appropriate point
+  in time on the *Dive Profile* panel and indicating the cylinder to which the change was made. After
+  right-clicking, follow the context menu to "Add gas change" and select the appropriate cylinder from
+  those defined during the first step, above (see image below). If the
+  *tank bar* button in the ttolbar has been activated, the cylinder switches are also indicated in the
+  tank bar.
+
+Having performed these tasks, _Subsurface_ indicates the appropriate use of cylinders in the dive profile.
+Below is a multi-cylinder dive, starting off with EAN28, then changing cylinders to EAN50 after 26 minutes
+to perform decompression.
+
+image::images/multicylinder_dive.jpg["FIGURE: Multicylinder profile",align="center"]
+
 ==== Sidemount dives
 
-_Subsurface_ easily handles dives involving more than one cylinder. Sidemount dive logging involves
-three steps:
+Sidemount diving is just another form of multicylinder diving, often with both or all cylinders having
+the same gas mixture. Although it is a popular configuration for cave divers, sidemount
+diving can be performed by recreational divers who have completed the appropriate training. Sidemount
+dive logging involves, exactly as with multicylinder dives, above, three steps:
 
-- *During the dive, recording cylinder switch events*. Since sidemount diving normally involves two
+- *During the dive, record cylinder switch events*. Since sidemount diving normally involves two
   cylinders with air or with the same gas mixture, _Subsurface_ distinguishes among these different
   cylinders. In contrast, most dive computers that allow gas switching only distinguish among different
   _gases_ used, not among different _cylinders_ used. This means that when sidemount dives are downloaded
@@ -1666,6 +1701,38 @@ image::images/sidemount1.jpg["FIGURE: Sidemount profile",align="center"]
 
 This section gives an example of the versatility of _Subsurface_ as a give logging tool.
 
+
+[[S_sSCR_dives]]
+==== Semi-closed circuit rebreather (SCR) dives
+
+[icon="images/halcyon_RB80.jpg"]
+[NOTE]
+Passive semi-closed rebreathers (pSCR) comprise a technical advance in diving equipment that
+recirculates the breathing gas that a diver breathes, while removing carbon dioxide from
+the exhaled gas. While a small amount (typically a tenth) of the exhaled breathing gas is released into the water,
+a small amount of fresh gas is released from the back gas cylinder (typically containing nitrox).
+A diver, using a single cylinder of breathing gas can therefore dive for much longer periods than
+using a recreational open-circuit configuration. With pSCR equipment, a very small amount of breathing
+gas is released every time the breather inhales. With active SCR (aSCR) equipment, in contrast, a small amount of
+breathing gas is released continuously from the back cylinder.
+
+To log pSCR dives, no special procedures are required, just the normal steps outlined above:
+
+- Select pSCR in the _Dive Mode_ dropdown list on the *Dive Info* panel.
+
+- pSCR diving often involves gas changes, requiring an additional cylinder.
+  Define all the appropriate cylinders as described above
+  and indicate the cylinder/gas changes as described above in the section on xref:S_MulticylinderDives[multicylinder dives].
+
+If a pSCR _Dive Mode_ has been selected, the dive ceiling for pSCR dives is adjusted for the oxygen
+drop accross the mouthpiece which often requires longer decompression periods. Below is a dive profile
+of a pSCR dive using EAN36 on the back cylinder and oxygen for decompression. Note that this dive lasted
+over two hours.
+
+image::images/pSCR_profile.jpg["FIGURE: pSCR profile",align="center"]
+
+
+
 [[S_CCR_dives]]
 ==== Closed circuit rebreather (CCR) dives
 
@@ -1842,7 +1909,7 @@ different colours:
 
 The profile also includes depth readings for the peaks and troughs in the graph.
 Thus, users should see the depth of the deepest point and other peaks. Mean depth
-is marked with a horizontal red line.
+is plotted as a grey line, indicating mean dive depth up to a particular moment during the dive.
 
 [icon="images/icons/scale.jpg"]
 [NOTE]
@@ -2698,8 +2765,10 @@ This panel allows two type of selections:
 
 ** _Show unused cylinders in Equipment Tab_: This checkbox allows display of information about unused cylinders when viewing the *Equipment Tab*. Conversely, if this box is not checked, and any cylinders entered using the *Equipment Tab* are not used (e.g. there was no gas switch to such a cylinder), then these cylinders are omitted from that list.
 
-** _Show average depth_: Activating this checkbox causes _Subsurface_ to draw a red line across
-   the dive profile, indicating the mean depth of the dive.
+** _Show average depth_: Activating this checkbox causes _Subsurface_ to draw a grey line across
+   the dive profile, indicating the mean depth of the dive up to a particular point in time during
+   that dive. Normally this is a u-shaped line indicating the deepest average depth just before the
+   ascent.
 
 * *Misc*: