pgsql: Adjust extract(epoch) example to clarify that it includes

Log Message:
-----------
Adjust extract(epoch) example to clarify that it includes fractional
seconds, per gripe from Richard Neill.  Also, add a cross-reference to
the to_timestamp function.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        func.sgml (r1.484 -> r1.485)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/func.sgml?r1=1.484&r2=1.485)