pgsql: Document change in large object snapshotting behavior as a

Log Message:
-----------
Document change in large object snapshotting behavior as a version
compatibility issue.

Modified Files:
--------------
pgsql/doc/src/sgml:
release.sgml (r1.374 -> r1.375)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/release.sgml.diff?r1=1.374&r2=1.375)