pgsql: Adjust comment about %t and %s to cover %m as well.

Log Message:
-----------
Adjust comment about %t and %s to cover %m as well. Some trivial
code beautification too.

Modified Files:
--------------
pgsql/src/backend/utils/error:
elog.c (r1.160 -> r1.161)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/error/elog.c.diff?r1=1.160&r2=1.161)