Docs/mm/damon/design: document quota goal self-tuning

Update DAMON design doc to explain the quota goal self-tuning, which can
be used by setting the goal's metric to metrics that kernel can
self-retrieve.

Link: https://lkml.kernel.org/r/20240219194431.159606-16-sj@kernel.org


Signed-off-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>