man: clarify the 'random number generator must be seeded' requirement

The manual pages require for some API functions that the 'random number
generator must be seeded' before calling the function.  Initially, this
was meant literally, i.e. the OpenSSL CSPRNG had to be seeded manually
before calling these functions.

Since version 1.1.1, the CSPRNG is seeded automatically on first use,
so it's not the responsibility of the programmer anymore.  Still, he
needs to be aware that the seeding might fail.

Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Tomas Mraz <tmraz@fedoraproject.org>
(Merged from https://github.com/openssl/openssl/pull/9257)