[gtksourceview] docs: document another change for 3.99.1

[Sébastien Wilmet <swilmet src gnome org>]

commit 52d552192ea4b43c8e353fa6371616b76c40214f
Author: Sébastien Wilmet <swilmet gnome org>
Date:   Sun Nov 13 17:56:57 2016 +0100

    docs: document another change for 3.99.1

 docs/reference/porting-guide.xml |    7 +++++++
 1 files changed, 7 insertions(+), 0 deletions(-)
---
diff --git a/docs/reference/porting-guide.xml b/docs/reference/porting-guide.xml
index 3defbae..f11b21a 100644
--- a/docs/reference/porting-guide.xml
+++ b/docs/reference/porting-guide.xml
@@ -63,6 +63,13 @@
       </listitem>
       <listitem>
         <para>
+          Only <code>&lt;gtksourceview/gtksource.h&gt;</code> can be included
+          directly. There were already warnings about it in GtkSourceView 3. The
+          warnings have been changed to errors.
+        </para>
+      </listitem>
+      <listitem>
+        <para>
           Only the version 2 of the <link linkend="lang-reference">GtkSourceView
           language definition file format</link> is supported (for *.lang files,
           used for syntax highlighting). The support for the version 1 has been