Document EVP_CIPHER_CTX_iv_length() return values

Document EVP_CIPHER_CTX_iv_length() return values

We aligned with upstream behavior. Let's document it properly.

Surprisingly, OpenSSL 1.1 half-assed the docs: two parts of the manual
contradict each other. The part getting EVP_CIPHER_CTX_iv_length() right,
incorrectly documents possible -1 return value to EVP_CIPHER_iv_length().

OpenSSL 3 documentation improvement efforts seem to have tried to address
this issue with the result that the manual is now entirely wrong when it
comes to the EVP_CIPHER_CTX_iv_length() replacement. Par for the course.