makedev(9): drop an additional note about cdevpriv dtors

These were previously somewhat safe to call destroy_dev(9), but will now
also cause a deadlock in the same fashion that d_close doing so would
previously. Amend the note to point it out, in case it's useful for
someone.

Reviewed by: imp, kib, markj
Differential Revision: https://reviews.freebsd.org/D53439