[gtkmm-documentation] Add description of the keep_return parameter in _WRAP_VFUNC

[Kjell Ahlstedt <kjellahl src gnome org>]

commit 0fe3c41e81503d113806bb45a0826732c267f240
Author: Kjell Ahlstedt <kjell ahlstedt bredband net>
Date:   Tue Mar 17 14:59:43 2015 +0100

    Add description of the keep_return parameter in _WRAP_VFUNC
    
    * docs/tutorial/C/index-in.docbook: "The .hg and .ccg files" section:
    Add description of the keep_return parameter in _WRAP_VFUNC. Bug #705124.

 docs/tutorial/C/index-in.docbook |   12 +++++++++++-
 1 files changed, 11 insertions(+), 1 deletions(-)
---
diff --git a/docs/tutorial/C/index-in.docbook b/docs/tutorial/C/index-in.docbook
index bfb4735..9b906d7 100644
--- a/docs/tutorial/C/index-in.docbook
+++ b/docs/tutorial/C/index-in.docbook
@@ -9597,7 +9597,9 @@ _WRAP_VFUNC(SizeRequestMode get_request_mode() const, get_request_mode)
 <para>The C function (e.g. <function>get_request_mode</function>) is described
     more fully in the <filename>*_vfuncs.defs</filename> file, and the
     <filename>convert*.m4</filename> files contain the necessary conversion from
-    the C++ parameter type to the C parameter type.</para>
+    the C++ parameter type to the C parameter type. Conversions can also be
+    written in the .hg file. Virtual functions often require special conversions
+    that are best kept local to the .hg file where they are used.</para>
 <para>There are some optional extra arguments:
 <variablelist>
     <varlistentry>
@@ -9618,6 +9620,14 @@ _WRAP_VFUNC(SizeRequestMode get_request_mode() const, get_request_mode)
         </listitem>
     </varlistentry>
     <varlistentry>
+        <term>keep_return</term>
+        <listitem>
+            <para>Keep a copy of the return value in the C callback function,
+                in case the calling C function does not expect to get its own
+                reference.</para>
+        </listitem>
+    </varlistentry>
+    <varlistentry>
         <term>errthrow</term>
         <listitem>
             <para>Use the last GError** parameter of the C virtual function (if