random(4): update Fortuna generator Chacha20 documentation.

The implementation was landed in r344913 and has had some bake time (at
least on my personal systems). There is some discussion of the motivation
for defaulting to this cipher as a PRF in the commit log for r344913.

Administrators retain the prior (AES-ICM) mode of operation by default.
The new mode may be used by setting the 'kern.random.use_chacha20_cipher'
tunable to "1" in loader.conf(5).

(cherry picked from commit 68b97d40fbe826585813f05042209db5490dbe08)