pgsql: Document that xact commit/abort stats are collected if any stat

Log Message:
-----------
Document that xact commit/abort stats are collected if any stat variable
is set.

Modified Files:
--------------
pgsql/doc/src/sgml:
monitoring.sgml (r1.29 -> r1.30)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/monitoring.sgml.diff?r1=1.29&r2=1.30)