* lisp/mail/rmailsum.el (rmail-summary-message-parents-vector)
(rmail-summary-message-descendants-vector): Doc fixes.
(rmail-summary-message-descendants-vector): New variable.
(rmail-summary-fill-message-parents-and-descs-vectors): Renamed
from 'rmail-summary-fill-message-parents-vector' and rewritten.
(rmail-summary-direct-descendants): Function deleted.