Document RAND_DRBG fork-safety locking model

Add some more exposition on why unlocked access to the global rand_fork_count
is safe, and provide a comment for the struct rand_drbg_st fork_count field.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
(Merged from https://github.com/openssl/openssl/pull/4110)