Doc: improve discussion of plpgsql's GET DIAGNOSTICS, other minor fixes.

9.4 added a second description of GET DIAGNOSTICS that was totally
independent of the existing one, resulting in each description lying to the
extent that it claimed the set of status items it described was complete.
Fix that, and do some minor markup improvement.

Also some other small fixes per bug #14258 from Dilian Palauzov.

Discussion: <20160718181437.1414.40802@wrigleys.postgresql.org>

doc/src/sgml/release-9.4.sgml

@@ -7308,7 +7308,7 @@ Branch: REL9_4_STABLE [c2b06ab17] 2015-01-30 22:45:58 -0500
       <listitem>
        <para>
         Add ability to retrieve the current PL/PgSQL call stack
-        using <link linkend="plpgsql-get-diagnostics-context"><command>GET
+        using <link linkend="plpgsql-call-stack"><command>GET
         DIAGNOSTICS</></link>
         (Pavel Stehule, Stephen Frost)
        </para>