Add documentation for "NULL" cipher and digest algorithms.

Fixes #20340

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from

Add documentation for "NULL" cipher and digest algorithms.

Fixes #20340

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/20356)

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