Document RNG design in random.h

Summary:
Partial backport of Core PR14955
https://github.com/bitcoin/bitcoin/pull/14955/commits/223de8d94d6522f795ec3c2e7db27469f24aa68c

Depends on D4400

Test Plan:

make check

Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: Fabien, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D4401