minor #8575 Clarifying usage of delivery_whitelist (tomhv)

This PR was submitted for the 3.3 branch but it was merged into the 2.7 branch instead (closes #8575).

Discussion
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Clarifying usage of delivery_whitelist

There was nothing to suggest that `delivery_whitelist` could not be used on its own. We had to find this out by trial an error.

Hopefully, this change (or better equivalent) will help other people.

Cheers.

Commits
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ceabda0 Clarifying usage of delivery_whitelist

Author: weaverryan

email/dev_environment.rst

@@ -191,6 +191,10 @@ In the above example all email messages will be redirected to ``dev@example.com`
 and messages sent to the ``admin@mydomain.com`` address or to any email address
 belonging to the domain ``specialdomain.com`` will also be delivered as normal.
 
+.. caution::
+
+    The ``delivery_whitelist`` option is ignored unless the ``delivery_addresses`` option is defined.
+
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