[gtkmm-documentation] Add description of voidreturn parameter in _WRAP_METHOD_DOCS_ONLY

[Kjell Ahlstedt <kjellahl src gnome org>]

commit 63c617fa549781fc8866e171a1ef212f3ad5d3cb
Author: Kjell Ahlstedt <kjellahlstedt gmail com>
Date:   Wed Nov 15 16:20:24 2017 +0100

    Add description of voidreturn parameter in _WRAP_METHOD_DOCS_ONLY
    
    Bug 787978

 docs/tutorial/C/index-in.docbook |    8 ++++++++
 1 files changed, 8 insertions(+), 0 deletions(-)
---
diff --git a/docs/tutorial/C/index-in.docbook b/docs/tutorial/C/index-in.docbook
index f01b803..3c8e303 100644
--- a/docs/tutorial/C/index-in.docbook
+++ b/docs/tutorial/C/index-in.docbook
@@ -10007,6 +10007,14 @@ _WRAP_METHOD_DOCS_ONLY(gtk_container_add)
                 the @newin command generated from the C documentation.</para>
         </listitem>
     </varlistentry>
+    <varlistentry>
+        <term>voidreturn</term>
+        <listitem>
+            <para>Don't include a @return Doxygen command in the documentation.
+                Useful if the wrapped C function returns a value, but the corresponding
+                C++ method returns <type>void</type>.</para>
+        </listitem>
+    </varlistentry>
 </variablelist>
 </para>
 </sect3>