mailman shell banner should mention about commit()

ritwikrequested to merge
(removed):i658 into master

Regarding #658 (closed) . Changed the default banner which was:

Welcome to the GNU Mailman shell

To:

Welcome to the GNU Mailman shell
Use commit() to commit changes
Use abort() to discard changes since the last commit.
Exit with ctrl+D does an implicit commit() but exit() does not.
Edited by ritwik

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