pgsql: Add mention that preload_libraries check the magic block, so

Log Message:
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Add mention that preload_libraries check the magic block, so
non-PostgreSQL libraries cannot be loaded using this capability.

Modified Files:
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pgsql/doc/src/sgml:
config.sgml (r1.61 -> r1.62)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/config.sgml.diff?r1=1.61&r2=1.62)