pgsql: Forbid using pg_xlogfile_name() and pg_xlogfile_name_offset()

Log Message:
-----------
Forbid using pg_xlogfile_name() and pg_xlogfile_name_offset() during
recovery. We might want to relax this in the future, but ThisTimeLineID
isn't currently correct in backends during recovery, so the filename
returned was wrong.

Modified Files:
--------------
pgsql/doc/src/sgml:
func.sgml (r1.512 -> r1.513)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/func.sgml?r1=1.512&r2=1.513)
pgsql/src/backend/access/transam:
xlog.c (r1.389 -> r1.390)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xlog.c?r1=1.389&r2=1.390)