[gtk-doc] help: Document the Stability tag for documentation comments

[Stefan Kost <stefkost src gnome org>]

commit 837a92fbd3c43e0c83de95e83efba1e06b9973a9
Author: Philip Withnall <philip withnall collabora co uk>
Date:   Thu Jun 25 08:48:03 2015 +0100

    help: Document the Stability tag for documentation comments
    
    It’s been a FIXME for too long.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=751479

 help/manual/C/index.docbook |   41 +++++++++++++++++++++++++++++++++++++++--
 1 files changed, 39 insertions(+), 2 deletions(-)
---
diff --git a/help/manual/C/index.docbook b/help/manual/C/index.docbook
index 9564620..e86de4c 100644
--- a/help/manual/C/index.docbook
+++ b/help/manual/C/index.docbook
@@ -1084,9 +1084,46 @@ gtkdoc-fixxref --module=$(DOC_MODULE) --module-dir=html
         </variablelist>
 
         <para>
-          (FIXME : Stability information)
+          You can also add stability information to all documentation elements
+          to indicate whether API stability is guaranteed for them for all
+          future minor releases of the project.
         </para>
-        
+
+        <para>
+          The default stability level for all documentation elements can be set
+          by passing the <option>--default-stability</option> argument to
+          <application>gtkdoc-mkdb</application> with one of the values below.
+        </para>
+
+        <variablelist><title>Stability Tags</title>
+          <varlistentry><term>Stability: Stable</term>
+            <listitem>
+              <para>
+                Mark the element as stable. This is for public APIs which are
+                guaranteed to remain stable for all future minor releases of the
+                project.
+              </para>
+            </listitem>
+          </varlistentry>
+          <varlistentry><term>Stability: Unstable</term>
+            <listitem>
+              <para>
+                Mark the element as unstable. This is for public APIs which are
+                released as a preview before being stabilised.
+              </para>
+            </listitem>
+          </varlistentry>
+          <varlistentry><term>Stability: Private</term>
+            <listitem>
+              <para>
+                Mark the element as private. This is for interfaces which can be
+                used by tightly coupled modules, but not by arbitrary third
+                parties.
+              </para>
+            </listitem>
+          </varlistentry>
+        </variablelist>
+
         <example><title>General tags</title>
           <programlisting><![CDATA[
 /**