pgsql: Document that the "-P" option to pg_autovacuum is insecure on

Log Message:
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Document that the "-P" option to pg_autovacuum is insecure on many
platforms, and suggest using ~/.pgpass instead.

Modified Files:
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pgsql/contrib/pg_autovacuum:
README.pg_autovacuum (r1.12 -> r1.13)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/pg_autovacuum/README.pg_autovacuum.diff?r1=1.12&r2=1.13)