Description tweak for vacuumdb

Hi

Like the docs say, vacuumdb is a "wrapper around the SQL command VACUUM"
which I used to use in dim-and-distant pre-autovacuum days came for cronjobs,
but until messing around with pg_upgrade recently I hadn't really had much
use for it. Anyway, the docs also say "There is no effective difference
between vacuuming and analyzing databases via this utility and via other
methods for accessing the server", which IMHO seems a bit out-of-date as
it now does two things which you can't do directly via e.g. psql:

- generate statistics in stages (9.4)
- parallel vacuum (9.5)

Attached patches (for 9.4 and 9.5/HEAD) update the description to make
clear that it now does a bit more than just execute a single command.

Regards

Ian Barwick

--
Ian Barwick http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services