random(4): Fortuna: Update concurrent generation documentation

The knob added in r349154 remains "disabled." The commit message from that
revision and associated code comment describe the rationale, implementation,
and motivation for the new option in detail.

For end-users: if you enable this, the result is just as secure.
The benefit is a faster, more responsive system when processes
produce significant demand on random(4).

As mentioned in the earlier commit, the prior behavior may be enabled by
setting the kern.random.fortuna.concurrent_read="1" knob in loader.conf(5).

This scales the random generation side of random(4) somewhat, although there
is still a global mutex being shared by all cores and rand_harvestq; the
situation is generally much better than it was before on small CPU systems,
but do not expect miracles on 256-core systems running 256-thread full-rate
random(4) read.

(cherry picked from commit 548dca90ae2fc3c0900c94a97e89aa97d6c36eae)