random(4): Remove "EXPERIMENTAL" verbiage from concurrent operation

No functional change.

Add a verbose comment giving an example side-by-side comparison between the
prior and Concurrent modes of Fortuna, and why one should believe they
produce the same result.

The intent is to flip this on by default prior to 13.0, so testing is
encouraged. To enable, add the following to loader.conf:

kern.random.fortuna.concurrent_read="1"

The intent is also to flip the default blockcipher to the faster Chacha-20
prior to 13.0, so testing of that mode of operation is also appreciated.
To enable, add the following to loader.conf:

kern.random.use_chacha20_cipher="1"

Approved by: secteam(implicit)