[wwwdocs] Update -Wnonnull description

[Marek Polacek]

Now that -Wnonnull-comare has been split out of -Wnonnull, we should also
update the porting_to text.  Is this sufficient?

Index: porting_to.html
===================================================================
RCS file: /cvs/gcc/wwwdocs/htdocs/gcc-6/porting_to.html,v
retrieving revision 1.15
diff -u -r1.15 porting_to.html
--- porting_to.html	23 Feb 2016 10:13:29 -0000	1.15
+++ porting_to.html	25 Feb 2016 12:17:00 -0000
@@ -388,12 +388,12 @@
 the indentation of the source was fixed instead.
 </p>
 
-<h2>Enhanced <code>-Wnonnull</code></h2>
+<h2>-Wnonnull-compare</h2>
 <p>
-The <code>-Wnonnull</code> warning has been improved so that it also warns
-about comparing parameters declared as <code>nonnull</code> with
-<code>NULL</code>.  For example, the compiler will warn about the
-following code:
+A new warning <code>-Wnonnull-compare</code> was added to <code>-Wall</code>.
+It warns about comparing parameters declared as <code>nonnull</code> with
+<code>NULL</code>.  For example, the compiler will now warn about the following
+code:
 </p>
 
 <pre><code>

	Marek