pgsql: Add documentation on how statistics are used by the planner.

Log Message:
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Add documentation on how statistics are used by the planner.

Mark Kirkwood

Modified Files:
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    pgsql/doc/src/sgml:
        filelist.sgml (r1.42 -> r1.43)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/filelist.sgml.diff?r1=1.42&r2=1.43)
        postgres.sgml (r1.74 -> r1.75)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/postgres.sgml.diff?r1=1.74&r2=1.75)

Added Files:
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    pgsql/doc/src/sgml:
        planstats.sgml (r1.1)

(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/planstats.sgml?rev=1.1&content-type=text/x-cvsweb-markup)