[Part of bug #1599254] Add suggestion to Mailbox docs to use Maildir, and warn user to lock/unlock mailboxes when modifying them
[Part of bug #1599254] Add suggestion to Mailbox docs to use Maildir, and warn user to lock/unlock mailboxes when modifying them

sk-/python2.7-type-annotator,sk-/python2.7-type-annotator,sk-/python2.7-type-annotator