Update d2i_PrivateKey documentation

Reviewed-by: NPaul Yang <yang.yang@baishancloud.com>
Reviewed-by: NMatt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/8168)

(cherry picked from commit 1980ce45d6bdd2b57df7003d6b56b5df560b9064)

doc/man3/d2i_PrivateKey.pod

@@ -50,15 +50,19 @@ If the B<*a> is not NULL when calling d2i_PrivateKey() or d2i_AutoPrivateKey()
 (i.e. an existing structure is being reused) and the key format is PKCS#8
 then B<*a> will be freed and replaced on a successful call.
 
+To decode a key with type B<EVP_PKEY_EC>, d2i_PublicKey() requires B<*a> to be
+a non-NULL EVP_PKEY structure assigned an EC_KEY structure referencing the proper
+EC_GROUP.
+
 =head1 RETURN VALUES
 
-d2i_PrivateKey() and d2i_AutoPrivateKey() return a valid B<EVP_KEY> structure
-or B<NULL> if an error occurs. The error code can be obtained by calling
-L<ERR_get_error(3)>.
+The d2i_PrivateKey(), d2i_AutoPrivateKey(), d2i_PrivateKey_bio(), d2i_PrivateKey_fp(),
+and d2i_PublicKey() functions return a valid B<EVP_KEY> structure or B<NULL> if an
+error occurs. The error code can be obtained by calling L<ERR_get_error(3)>.
 
-i2d_PrivateKey() returns the number of bytes successfully encoded or a
-negative value if an error occurs. The error code can be obtained by calling
-L<ERR_get_error(3)>.
+i2d_PrivateKey() and i2d_PublicKey() return the number of bytes successfully
+encoded or a negative value if an error occurs. The error code can be obtained
+by calling L<ERR_get_error(3)>.
 
 =head1 SEE ALSO
 
@@ -67,7 +71,7 @@ L<d2i_PKCS8PrivateKey_bio(3)>
 
 =head1 COPYRIGHT
 
-Copyright 2017-2018 The OpenSSL Project Authors. All Rights Reserved.
+Copyright 2017-2019 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