Update documentation on stream email address

[alya]

We should update https://zulip.com/help/message-a-stream-by-email once #20985 is completed.

The documentation should also describe in more detail how .include-footer behaves, to avoid the behavior being unexpected for users (#22585).

[zulipbot]

Hello @zulip/server-user-docs members, this issue was labeled with the "area: documentation (user)" label, so you may want to check it out!

[alya]

I think the remaining part here is to review #22585 and see if there are helpful details we should add on https://zulip.com/help/message-a-stream-by-email. @drrosa perhaps you could add this to your todo list? Not a release goal.

[drrosa]

@zulipbot claim

[zulipbot]

@drrosa You have been unassigned from this issue because you have not made any updates for over 14 days. Please feel free to reclaim the issue if you decide to pick up again. Thanks!