Fix documentation for the privileges required for replication functions.

Previously it's documented that use of replication functions is
restricted to superusers. This is true for the functions which
use replication origin, but not for pg_logicl_emit_message() and
functions which use replication slot. For example, not only
superusers but also users with REPLICATION privilege is allowed
to use the functions for replication slot. This commit fixes
the documentation for the privileges required for those replication
functions.

Back-patch to 9.4 (all supported versions).

Author: Matsumura Ryo
Discussion: https://postgr.es/m/03040DFF97E6E54E88D3BFEE5F5480F74ABA6E16@G01JPEXMBYT04

doc/src/sgml/func.sgml

@@ -20337,8 +20337,10 @@ postgres=# SELECT * FROM pg_walfile_name_offset(pg_stop_backup());
     See <xref linkend="streaming-replication"/>,
     <xref linkend="streaming-replication-slots"/>, and
     <xref linkend="replication-origins"/>
-    for information about the underlying features.  Use of these
-    functions is restricted to superusers.
+    for information about the underlying features.
+    Use of functions for replication origin is restricted to superusers.
+    Use of functions for replication slot is restricted to superusers
+    and users having <literal>REPLICATION</literal> privilege.
    </para>
 
    <para>