Fix documentation comment for pg_md5_hash

Fix documentation comment for pg_md5_hash

Commit b69aba74578 added the errstr parameter to pg_md5_hash but
missed updating the synopsis in the documentation comment.  The
follow-up commit 587de223f03 added the parameter to the list of
outputs.  The returnvalue had been changed from integer to bool
before that but remained in the synopsis.  This fixes both.

Author: Tatsuro Yamada <[email protected]>
Discussion: https://postgr.es/m/TYYPR01MB82313576150CC86084A122CD9E292@TYYPR01MB8231.jpnprd01.prod.outlook.com