pgsql: Remove caveat about avoiding cross-type operators in constraints

Log Message:
-----------
Remove caveat about avoiding cross-type operators in constraints intended
for use with constraint exclusion. We can prove those cases now...

Modified Files:
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pgsql/doc/src/sgml:
ddl.sgml (r1.69 -> r1.70)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ddl.sgml.diff?r1=1.69&r2=1.70)