pgsql: Improve documentation/warning when --with-libxml is not used in

Log Message:
-----------
Improve documentation/warning when --with-libxml is not used in the
installation.

Modified Files:
--------------
pgsql/doc/src/sgml:
datatype.sgml (r1.193 -> r1.194)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/datatype.sgml.diff?r1=1.193&r2=1.194)
func.sgml (r1.374 -> r1.375)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/func.sgml.diff?r1=1.374&r2=1.375)
pgsql/src/backend/utils/adt:
xml.c (r1.39 -> r1.40)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/xml.c.diff?r1=1.39&r2=1.40)