pgsql: Fix thinko in log message for "sameuser" ident map mismatch: the

Log Message:
-----------
Fix thinko in log message for "sameuser" ident map mismatch: the provided
and authenticated usernames were swapped.  Reported by Bryan Henderson
in bug #5386.

Also clean up poorly-maintained header comment for this function.

Modified Files:
--------------
    pgsql/src/backend/libpq:
        hba.c (r1.203 -> r1.204)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/libpq/hba.c?r1=1.203&r2=1.204)