Add documentation for key validation that indicates the difference between the

Add documentation for key validation that indicates the difference between the
EVP_PKEY_XXX_check() calls for the default and fips providers.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/18235)