pgsql: Improve documentation around autovacuum-related storage paramete

Improve documentation around autovacuum-related storage parameters.

These were discussed in three different sections of the manual, which
unsurprisingly had diverged over time; and the descriptions of individual
variables lacked stylistic consistency even within each section (and
frequently weren't in very good English anyway). Clean up the mess, and
remove some of the redundant information in hopes that future additions
will be less likely to re-introduce inconsistency. For instance I see
no need for maintenance.sgml to include its very own list of all the
autovacuum storage parameters, especially since that list was already
incomplete.

Branch
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REL9_5_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/bcb8f96e6775c649564ac0fb946ab6a1629ff969

Modified Files
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doc/src/sgml/config.sgml | 65 +++++++++++-----------
doc/src/sgml/maintenance.sgml | 53 ++++++------------
doc/src/sgml/ref/create_table.sgml | 104 +++++++++++++++++++-----------------
3 files changed, 106 insertions(+), 116 deletions(-)