doc: Update note about source code formatting

Update the note about why not to use // comments, even though it's now
technically supported.

The note about variable declarations was dropped here because it's
addressed more properly later in the chapter.

Discussion: https://www.postgresql.org/message-id/flat/156924954640.1117.6309209869705522549%40wrigleys.postgresql.org

doc/src/sgml/sources.sgml

@@ -26,9 +26,9 @@
    </para>
 
    <para>
-    Do not use C++ style comments (<literal>//</literal> comments).  Strict ANSI C
+    To maintain a consistent coding style, do not use C++ style comments
-    compilers do not accept them.  For the same reason, do not use C++
+    (<literal>//</literal> comments).  <application>pgindent</application>
-    extensions such as declaring new variables mid-block.
+    will replace them with <literal>/* ... */</literal>.
    </para>
 
    <para>