pgsql: Add documentation mention that FSM == Free Space Map, so the

Log Message:
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Add documentation mention that FSM == Free Space Map, so the config
parameter name makes more sense.

Modified Files:
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pgsql/doc/src/sgml:
config.sgml (r1.151 -> r1.152)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/config.sgml?r1=1.151&r2=1.152)