libpq documentation cleanups (repost 3)

The attached patch is a collection of libpq documentation cleanups
recommended in a list of changes emailed to me by Leslie S Satenstein.

Leslie found a number of places our libpq documentation that were unclear
or awkward, and this diff generated by me attempts to address them.

I have already updated the documentation proofreading wiki:

	http://wiki.postgresql.org/wiki/Documentation_Proofreading

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

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