Document that exdata for ENGINES is deprecated.

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

doc/man3/CRYPTO_get_ex_new_index.pod

@@ -163,9 +163,12 @@ dup_func() should return 0 for failure and 1 for success.
 =head1 HISTORY
 
 CRYPTO_alloc_ex_data() was added in OpenSSL 3.0.
+
 The signature of the dup_func() callback was changed in OpenSSL 3.0 to use the
 type B<void **> for B<from_d>.  Previously this parameter was of type B<void *>.
 
+Support for ENGINE "exdata" was deprecated in OpenSSL 3.0.
+
 =head1 COPYRIGHT
 
 Copyright 2015-2020 The OpenSSL Project Authors. All Rights Reserved.