rand: add a note about a potentially misleading code analyzer warning.
rand: add a note about a potentially misleading code analyzer warning.

When seeding from a parent DRBG, the pointer to the child is used as
additional data.  This triggers static code analysers.  Rearrange and
expand the comments to make this more obvious.

Reviewed-by: Matthias St. Pierre <1910231d24f125aceac7850dc92279eee8d52323@ncp-e.com>
(Merged from https://github.com/openssl/openssl/pull/12724)

openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl