Document that EVP_get_cipherbyname() does not work for some new algorithm names.

Document that EVP_get_cipherbyname() does not work for some new algorithm names.

These algorithms were added to providers but have no const EVP_CIPHER*
mapping. Ciphers for SIV and CTS were previously only available via low level
function calls that are deprecated.

Reported by @reaperhulk.

Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/16414)