http://issues.liferay.com/browse/LRDOCS-497

http://issues.liferay.com/browse/LRDOCS-497

Added information to Message Boards section of User Guide covering distiniction between user subscriptions and mailing lists, the features of each, and how to enable them

Clarified section on mail subdomains and the difference in behavior when enabled/disabled; also added note that globally scoped message boards do not support user subscription notifications or mailing lists