doc: Mention the update of der data pointers in d2i/i2d

Fixes #15958

Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/15959)

doc/man3/d2i_PrivateKey.pod

@@ -86,6 +86,10 @@ functions described in L<d2i_X509(3)>.
 
 =head1 NOTES
 
+All the functions that operate on data in memory update the data pointer I<*pp>
+after a successful operation, just like the other d2i and i2d functions;
+see L<d2i_X509(3)>.
+
 All these functions use DER format and unencrypted keys. Applications wishing
 to encrypt or decrypt private keys should use other functions such as
 d2i_PKCS8PrivateKey() instead.

doc/man3/d2i_RSAPrivateKey.pod

@@ -265,8 +265,8 @@ Functions such as strlen() will B<not> return the correct length
 of the encoded structure.
 
 The ways that I<*ppin> and I<*ppout> are incremented after the operation
-can trap the unwary. See the B<WARNINGS> section for some common
-errors.
+can trap the unwary. See the B<WARNINGS> section in L<d2i_X509(3)> for some
+common errors.
 The reason for this-auto increment behaviour is to reflect a typical
 usage of ASN1 functions: after one structure is encoded or decoded
 another will be processed after it.