pgpass comment suggestion

There was a discussion months ago about helping people document the
pgpass columns, so I added a documentation sentence suggestion the use
of a comment.  Patch attached and applied to 9.1 and head.

-- 
  Bruce Momjian  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

  + It's impossible for everything to be true. +