pgsql: Marginal improvements in the wording of the autovacuum

Log Message:
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Marginal improvements in the wording of the autovacuum documentation:
be consistent about whether it's called a daemon or a subprocess, and
don't describe the autovacuum setting in exactly the same way as the
stats_start_collector setting, because that leaves people thinking (if
they aren't paying close attention) that autovacuum can't be changed
on the fly.

Modified Files:
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    pgsql/doc/src/sgml:
        config.sgml (r1.43 -> r1.44)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/config.sgml.diff?r1=1.43&r2=1.44)