Pull up following revision(s) (requested by riastradh in ticket #1007):

share/man/man9/rnd.9: revision 1.26
share/man/man9/rnd.9: revision 1.27

Update rnd(9) man page to reflect reality since netbsd-7.
- Note rndsource_setcb, RND_FLAG_HASCB, and rnd_add_data_sync.
- Note user's obligation to serialize access to each rndsource.

Simplify macro usage.

(martin)