I could not find this documented; probably my Google-fu is
lacking, but the flags you mentioned, A, D, and M, appear to
stand for "added", "deleted", and "modified", respectively, according
to the source code (in diffdir.py):
log.Info(_("A %s") %
(util.ufn(delta_path.get_relative_path())),
log.InfoCode.diff_file_new,
util.escape(delta_path.get_relative_path()))
(...)
log.Info(_("D %s") %
(util.ufn(sig_path.get_relative_path())),
log.InfoCode.diff_file_deleted,
util.escape(sig_path.get_relative_path()))
(...)
log.Info(_("M %s") %
(util.ufn(delta_path.get_relative_path())),
log.InfoCode.diff_file_changed,
util.escape(delta_path.get_relative_path()))