doc: deprecate ENGINE documentation

Reviewed-by: NRichard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/12226)

doc/man3/ENGINE_add.pod

@@ -46,6 +46,10 @@ ENGINE_unregister_digests
 
  #include <openssl/engine.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)>:
+
  ENGINE *ENGINE_get_first(void);
  ENGINE *ENGINE_get_last(void);
  ENGINE *ENGINE_get_next(ENGINE *e);
@@ -162,6 +166,9 @@ L<openssl_user_macros(7)>:
 
 =head1 DESCRIPTION
 
+All of the functions described on this page are deprecated.
+Applications should instead use the provider APIs.
+
 These functions create, manipulate, and use cryptographic modules in the
 form of B<ENGINE> objects. These objects act as containers for
 implementations of cryptographic algorithms, and support a
@@ -651,6 +658,8 @@ L<RAND_bytes(3)>, L<config(5)>
 
 =head1 HISTORY
 
+All of these functions were deprecated in OpenSSL 3.0.
+
 ENGINE_cleanup() was deprecated in OpenSSL 1.1.0 by the automatic cleanup
 done by OPENSSL_cleanup()
 and should not be used.