pgsql: Update read committed documentation to better explain undesirable

Log Message:
-----------
Update read committed documentation to better explain undesirable
behavior of concurrent commands in cases where rows are being added and
removed from matching query criteria.

Minor word-smithing.

Modified Files:
--------------
pgsql/doc/src/sgml:
mvcc.sgml (r2.69 -> r2.70)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/mvcc.sgml?r1=2.69&r2=2.70)