Update d2i_PrivateKey documentation

Reviewed-by: Paul Yang <[email protected]>
Reviewed-by: Matt Caswell <[email protected]>
(Merged from openssl#8168)

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 Apache License 2.0 (the "License").  You may not use
 this file except in compliance with the License.  You can obtain a copy