pgsql: Document the idea of creating a symbolic link in /tmp to prevent

Log Message:
-----------
Document the idea of creating a symbolic link in /tmp to prevent server
spoofing when the socket file has been moved.

Modified Files:
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pgsql/doc/src/sgml:
runtime.sgml (r1.403 -> r1.404)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/runtime.sgml?r1=1.403&r2=1.404)