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(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

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