pgsql: Fix documentation for the privileges required for replicationfu

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

Branch
------
REL9_6_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/44b973b71c9566b946bd2a3dd1570d4fed1547a8

Modified Files
--------------
doc/src/sgml/func.sgml | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)