Document X509_check_private_key and relative

Document two private key check functions:

X509_check_private_key
X509_REQ_check_private_key

Signed-off-by: Paul Yang <paulyang.inf@gmail.com>

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3614)

doc/man3/X509_check_private_key.pod

@@ -0,0 +1,54 @@
+=pod
+
+=head1 NAME
+
+X509_check_private_key, X509_REQ_check_private_key - check the consistency
+of a private key with the public key in an X509 certificate or certificate
+request.
+
+=head1 SYNOPSIS
+
+ #include <openssl/x509.h>
+
+ int X509_check_private_key(X509 *x, EVP_PKEY *k);
+
+ int X509_REQ_check_private_key(X509_REQ *x, EVP_PKEY *k);
+
+=head1 DESCRIPTION
+
+X509_check_private_key() function checks the consistency of private
+key B<k> with the public key in B<x>.
+
+X509_REQ_check_private_key() is equivalent to X509_check_private_key()
+except that B<x> represents a certificate request of structure B<X509_REQ>.
+
+=head1 RETURN VALUE
+
+X509_check_private_key() and X509_REQ_check_private_key() return 1 if
+the keys match each other, and 0 if not.
+
+If the key is invalid or an error occurred, the reason code can be
+obtained using L<ERR_get_error(3)>.
+
+=head1 BUGS
+
+The B<check_private_key> functions don't check if B<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 B<k>, it will
+return success.
+
+=head1 SEE ALSO
+
+L<ERR_get_error(3)>
+
+=head1 COPYRIGHT
+
+Copyright 2017 The OpenSSL Project Authors. All Rights Reserved.
+
+Licensed under the OpenSSL license (the "License").  You may not use
+this file except in compliance with the License.  You can obtain a copy
+in the file LICENSE in the source distribution or at
+L<https://www.openssl.org/source/license.html>.
+
+=cut