allow for the following <figure...><caption><comment …></caption>
This seems to already be the case. So closing.
See: https://github.com/cu-mkp/m-k-manuscript-data/blob/1ffc8ae75fa5c46dbe7ebac40b73acdb24e65b67/schema/ms-transcription.rng#L120-L125
<caption> allows text and the elements in the m.phrase pattern, of which <comment> is a member:
https://github.com/cu-mkp/m-k-manuscript-data/blob/1ffc8ae75fa5c46dbe7ebac40b73acdb24e65b67/schema/ms-transcription.rng#L597-L641