pgsql: Generalize width_bucket() documentation because b1/b2 can be

Log Message:
-----------
Generalize width_bucket() documentation because b1/b2 can be
interchanged.

Modified Files:
--------------
pgsql/doc/src/sgml:
func.sgml (r1.331 -> r1.332)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/func.sgml.diff?r1=1.331&r2=1.332)