doc: EVP_KDF document the semantic meaning of output

doc: EVP_KDF document the semantic meaning of output

Explicitely document what semantic meaning do various EVP_KDF
algorithms produce.

PBKDF2 produces cryptographic keys that are subject to cryptographic
security measures, for example as defined in NIST SP 800-132.

All other algorithms produce keying material, not subject to explicit
output length checks in any known standards.

Reviewed-by: Paul Dale <ppzgs1@gmail.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/25610)