pgsql: Update "encode" documentation to mention that 'escape' only

Log Message:
-----------
Update "encode" documentation to mention that 'escape' only changes null
bytes and backslashes, remove "ASCII" mention. Backpatch to 8.2.X.

Modified Files:
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pgsql/doc/src/sgml:
func.sgml (r1.368 -> r1.369)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/func.sgml.diff?r1=1.368&r2=1.369)