[gtkmm-documentation] Describe the optional err_return_value argument in _WRAP_VFUNC

[Kjell Ahlstedt <kjellahl src gnome org>]

commit bd16bab60983e38ae32ea76a4501212185269015
Author: Kjell Ahlstedt <kjell ahlstedt bredband net>
Date:   Thu Jun 2 12:57:49 2016 +0200

    Describe the optional err_return_value argument in _WRAP_VFUNC

 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 0184479..e346eb6 100644
--- a/docs/tutorial/C/index-in.docbook
+++ b/docs/tutorial/C/index-in.docbook
@@ -9938,7 +9938,17 @@ _WRAP_VFUNC(SizeRequestMode get_request_mode() const, get_request_mode)
     <varlistentry>
         <term>return_value &lt;value&gt;</term>
              <listitem>
-               <para>Defines non-default return value.</para>
+               <para>Defines a non-default return value.</para>
+             </listitem>
+    </varlistentry>
+    <varlistentry>
+        <term>err_return_value &lt;value&gt;</term>
+             <listitem>
+               <para>Defines a non-default return value, used only if the C++
+                 <function>something_vfunc()</function> function throws an exception
+                 which is propagated to the C callback function. If return_value is
+                 specified, but err_return_value is not, then return_value is used
+                 also when an exception is propagated.</para>
              </listitem>
     </varlistentry>
     <varlistentry>