Docs/mm/damon/design: document DAMOS apply intervals

Update DAMON design doc to explain about DAMOS apply intervals.

Link: https://lkml.kernel.org/r/20230916020945.47296-6-sj@kernel.org


Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>