Without enabling the Messages Tab in App Home settings, users see
"Sending messages to this app has been turned off" when trying to DM
the bot — even with all correct scopes and event subscriptions.
Add Step 5 (Enable the Messages Tab) between Event Subscriptions and
Install App, with a danger admonition. Also add troubleshooting entry
for this specific error message. Renumber subsequent steps (6→7→8→9).
Co-authored-by: Alberto Leal <mail4alberto@gmail.com>
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