pg_upgrade docs

This doc sentence about pg_upgrade is now inaccurate:

   If doing <option>--check</> with a running old server of a pre-9.1 version,
   and the old server is using a Unix-domain socket directory that is
   different from the default built into the new <productname>PostgreSQL</>
   installation, set <envar>PGHOST</> to point to the socket location of the
   old server.  (This is not relevant on Windows.)

The new detail is that this also affects non-live check and non-check
upgrades because pg_ctl -w doesn't work for pre-9.1 servers with the
socket in the current directory --- that was not known when this
documentation paragraph was written.

Applied doc patch attached.  The wording became pretty complex so I
tried to simplify it.

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

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