perf/x86: Add a few more comments

Add a few comments on the ->add(), ->del() and ->*_txn()
implementation.

Requested-by: Vince Weaver <vincent.weaver@maine.edu>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/n/tip-he3819318c245j7t5e1e22tr@git.kernel.org


Signed-off-by: Ingo Molnar <mingo@kernel.org>