rsa: document deprecated low level functions

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/11063)

doc/man3/RSA_check_key.pod

diff --git a/doc/man3/RSA_check_key.pod b/doc/man3/RSA_check_key.pod
index fb7bd23..137ec7e 100644
--- a/doc/man3/RSA_check_key.pod
+++ b/doc/man3/RSA_check_key.pod
@@ -8,12 +8,20 @@
 
  #include <openssl/rsa.h>
 
+Deprecated since OpenSSL 3.0, can be hidden entirely by defining
+B<OPENSSL_API_COMPAT> with a suitable version value, see
+L<openssl_user_macros(7)>:
+
  int RSA_check_key_ex(RSA *rsa, BN_GENCB *cb);
 
  int RSA_check_key(RSA *rsa);
 
 =head1 DESCRIPTION
 
+Both of the functions described on this page are deprecated.
+Applications should instead use L<EVP_PKEY_public_check(3)>,
+L<EVP_PKEY_private_check(3)> and L<EVP_PKEY_pairwise_check(3)>.
+
 RSA_check_key_ex() function validates RSA keys.
 It checks that B<p> and B<q> are
 in fact prime, and that B<n = p*q>.
@@ -70,6 +78,8 @@
 
 =head1 HISTORY
 
+All of these functions were deprecated in OpenSSL 3.0.
+
 RSA_check_key_ex() appeared after OpenSSL 1.0.2.
 
 =head1 COPYRIGHT