pgsql: Document that enabling asserts can _significantly_ slow down the

Log Message:
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Document that enabling asserts can _significantly_ slow down the server.
Back patch to 8.3.X.

Tags:
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REL8_3_STABLE

Modified Files:
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pgsql/doc/src/sgml:
installation.sgml (r1.300 -> r1.300.2.1)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/installation.sgml?r1=1.300&r2=1.300.2.1)