pgsql: Clarify documention workding for xip_list().

Log Message:
-----------
Clarify documention workding for xip_list().

Simon Riggs

Modified Files:
--------------
pgsql/doc/src/sgml:
func.sgml (r1.444 -> r1.445)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/func.sgml?r1=1.444&r2=1.445)