Document the deprecation of ERR_STATE and ERR_get_state()

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/9462)

CHANGES

@@ -9,6 +9,11 @@
 
  Changes between 1.1.1 and 3.0.0 [xx XXX xxxx]
 
+  *) Deprecated the public definition of ERR_STATE as well as the function
+     ERR_get_state().  This is done in preparation of making ERR_STATE an
+     opaque type.
+     [Richard Levitte]
+
   *) Added ERR functionality to give callers access to the stored function
      names that have replaced the older function code based functions.