pgsql: Provide better guidance for adjusting shared_buffers.

Log Message:
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Provide better guidance for adjusting shared_buffers.

This change was previously committed to HEAD, but the consensus seems to be
in favor of back-patching it.  I'm only backpatching as far as 8.3.X, however,
because it's not clear to me to what degree this advice applies to older
branches, and in any case our first advice to anyone attempting to tune those
versions is likely to be "upgrade".

Tags:
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REL8_4_STABLE

Modified Files:
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    pgsql/doc/src/sgml:
        config.sgml (r1.220.2.3 -> r1.220.2.4)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/config.sgml?r1=1.220.2.3&r2=1.220.2.4)