[gtk-doc] docs: document the limitations of the __GTK_DOC_IGNORE__ conditional

[Stefan Kost <stefkost src gnome org>]

commit 27a5c3df5ca4e46eb519fa95076efdc31cc98f18
Author: Stefan Sauer <ensonic users sf net>
Date:   Mon Apr 27 22:06:21 2015 +0200

    docs: document the limitations of the __GTK_DOC_IGNORE__ conditional
    
    See https://bugzilla.gnome.org/show_bug.cgi?id=708366

 help/manual/C/index.docbook |   11 ++++++++++-
 1 files changed, 10 insertions(+), 1 deletions(-)
---
diff --git a/help/manual/C/index.docbook b/help/manual/C/index.docbook
index d8ba14d..b799bb6 100644
--- a/help/manual/C/index.docbook
+++ b/help/manual/C/index.docbook
@@ -653,7 +653,7 @@ gtkdoc-fixxref --module=$(DOC_MODULE) --module-dir=html
     <para>
       The scanner can handle the majority of C headers fine. In the case of
       receiving warnings from the scanner that look like a special case, one can
-      hint GTK-Doc to skip over them.
+      hint GTK-Doc to skip over them. 
       <example><title>GTK-Doc comment block</title>
         <programlisting><![CDATA[
 #ifndef __GTK_DOC_IGNORE__
@@ -663,6 +663,15 @@ gtkdoc-fixxref --module=$(DOC_MODULE) --module-dir=html
       </example>
     </para>
 
+    <note>
+      <title>Limitations</title>
+      <para>
+        Note, that GTK-Doc's supports
+        <code>#ifndef(__GTK_DOC_IGNORE__)</code> but not 
+        <code>#if !defined(__GTK_DOC_IGNORE__)</code> or other combinations.
+      </para>
+    </note>
+
     <!--  -->
 
     <sect1 id="documenting_syntax">