+If this is the symbol `as-is', then Notmuch does not modify the
+value of `message-completion-alist'. This option has to be set to
+this value before `notmuch' is loaded, otherwise the modification
+to `message-completion-alist' may already have taken place. This
+setting obviously does not prevent `message-completion-alist'
+from being modified at all; the user or some third-party package
+may still modify it.
+
+Finally, if this is nil, then address completion is disabled."