+Destination for results (e.g., C<path/to/Maildir> or
+C<mboxcl2:path/to/mbox>). The format can be specified by adding a
+C<E<lt>formatE<gt>:> prefix with any of these values: C<maildir>,
+C<mboxrd>, C<mboxcl2>, C<mboxcl>, C<mboxo>, C<json>, C<jsonl>, or
+C<concatjson>.
+
+TODO: Provide description of formats?
+
+When a format isn't specified, it's chosen based on the destination.
+C<json> is used for the default destination (stdout), and C<maildir>
+is used for an existing directory or non-existing path.