Description:
------------
The $key parameter does not necessarily need to be a private key for the function to return true.  

The OpenSSL docs have a "BUGS" section for this function:

"The check_private_key functions don't check if k itself is indeed a private key or not. It merely compares the public materials (e.g. exponent and modulus of an RSA key) and/or key parameters (e.g. EC params of an EC key) of a key pair. So if you pass a public key to these functions in k, it will return success."
 - from https://www.openssl.org/docs/manmaster/man3/X509_check_private_key.html

This should be incorporated into our docs.