Correct sepgsql docs with regard to RLS

The sepgsql docs included a comment that PG doesn't support RLS.  That
is only true for versions prior to 9.5.

Update the docs for 9.5 and master to say that PG supports RLS but that
sepgsql does not yet.

Pointed out by Heikki.

Back-patch to 9.5

Author: sfrost

doc/src/sgml/sepgsql.sgml

@@ -726,8 +726,8 @@ ERROR:  SELinux: security policy violation
     <term>Row-level access control</term>
     <listitem>
      <para>
-      <productname>PostgreSQL</> does not support row-level access; therefore,
-      <filename>sepgsql</filename> does not support it either.
+      <productname>PostgreSQL</> supports row-level access, but
+      <filename>sepgsql</filename> does not.
      </para>
     </listitem>
    </varlistentry>