doc: minor corrections in random.cpp

Summary:

This should have been part of #17151.

Partial backport of core PR17270:
https://github.com/bitcoin/bitcoin/pull/17270/commits/b51bae1a5a4fa8ef7825dd1bb09e3f47f96d7a5a

Depends on D6157.

Test Plan: Read the comments.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6158