pgsql: Remove installation mention that integer timestamps is less

Log Message:
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Remove installation mention that integer timestamps is less stable that
floating point.

Modified Files:
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pgsql/doc/src/sgml:
installation.sgml (r1.282 -> r1.283)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/installation.sgml.diff?r1=1.282&r2=1.283)