small changes to autovacuum maintainance

Attached patch takes a swing at improving the autovacuum wording in
maintenance.sgml.

On a related note I was thinking to add a note in the runtime config section
for autovacuum_enabled that stats_start_collector and stats_row_level must be
on for enabled to work, would that still go in runtime.sgml ?

On a related note to that, I am wondering if it might be better to have the
postmaster issue a fatal error on startup if autovacuum is on and stats are
off, rather than the warning it does now. ISTM it is very easy to miss that
warning (since it is only sent to stderr and not stout afaict) and looking in
pg_settings there would be no indication that anything was wrong, other than
autovacuum being set to off (perhaps if it was set to 'error, stats disabled'
or some such it might be better?)

--
Robert Treat
Build A Brighter Lamp :: Linux Apache {middleware} PostgreSQL