Docs/mm/damon/design: document for intervals auto-tuning

Document the design of DAMON sampling and aggregation intervals
auto-tuning.

[sj@kernel.org: fix a typo on 'intervals auto-tuning' section]
  Link: https://lkml.kernel.org/r/20250305182744.56125-1-sj@kernel.org
Link: https://lkml.kernel.org/r/20250303221726.484227-7-sj@kernel.org


Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>