base: Tag API methods and variables in callback.hh

Change-Id: I4ff6d9914e6edb4e8e7b033aec4b26482e950cc0
Signed-off-by: Hoa Nguyen <hoanguyen@ucdavis.edu>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/32959
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>

src/base/callback.hh

@@ -39,6 +39,9 @@ class CallbackQueue : public std::list<std::function<void()>>
 
     using Base::Base;
 
+    /**
+     * @ingroup api_callback
+     */
     void
     process()
     {