Explain compatibility item about language names a bit more.

Since we've got an "open items" list item about this, apparently some
people are pretty worried about it.

In passing remove a lot of trailing whitespace.

doc/src/sgml/release-9.2.sgml

@@ -187,8 +187,11 @@
        </para>
 
        <para>
-        While unquoted language identifiers are still lowercased, double
+        While unquoted language identifiers are still lowercased, strings
-        and single-quoted identifiers are no longer down-cased.
+        and quoted identifiers are no longer forcibly down-cased.
+        Thus for example <literal>CREATE FUNCTION ... LANGUAGE 'C'</>
+        will no longer work; it must be spelled <literal>'c'</>, or better
+        omit the quotes.
        </para>
       </listitem>