Fix upgrading docs for RSA_private_encrypt/RSA_public_decrypt

Despite the name, these functions manipulate signatures, which means
that their replacements are the EVP_PKEY_sign/EVP_PKEY_verify family.
Signed-off-by: NRobbie Harwood <rharwood@redhat.com>
Reviewed-by: NTomas Mraz <tomas@openssl.org>
Reviewed-by: NMatt Caswell <matt@openssl.org>
Reviewed-by: NDmitry Belyavskiy <beldmit@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/15359)

doc/man3/RSA_private_encrypt.pod

@@ -21,9 +21,9 @@ L<openssl_user_macros(7)>:
 =head1 DESCRIPTION
 
 Both of the functions described on this page are deprecated.
-Applications should instead use L<EVP_PKEY_encrypt_init_ex(3)>,
-L<EVP_PKEY_encrypt(3)>, L<EVP_PKEY_decrypt_init_ex(3)> and
-L<EVP_PKEY_decrypt(3)>.
+Applications should instead use L<EVP_PKEY_sign_init_ex(3)>,
+L<EVP_PKEY_sign(3)>, L<EVP_PKEY_verify_init_ex(3)> and
+L<EVP_PKEY_verify(3)>.
 
 These functions handle RSA signatures at a low-level.