feat: Add hint for app password in case of 2FA.

Fixes https://github.com/nextcloud/documentation/issues/12460

Signed-off-by: tflidd <tflidd@aspekte.net>

user_manual/groupware/sync_thunderbird.rst

@@ -22,6 +22,8 @@ Contacts
 
 If you later want to add a new address book, you can redo all of those steps and only the books not already synchronized will be suggested.
 
+.. note:: If your account uses two-factor authentication, you need a :ref:`dedicated app password <managing_devices>` for login rather than your
+   regular password.
 
 Calendars
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