Docs/mm/damon/design: document vaddr support for migrate_{hot,cold}

Docs/mm/damon/design: document vaddr support for migrate_{hot,cold}

Document that the migrate_{hot,cold} schemes can be used by the vaddr
operations set.

Link: https://lkml.kernel.org/r/20250709005952.17776-11-bijan311@gmail.com
Co-developed-by: Ravi Shankar Jonnalagadda <ravis.opensrc@micron.com>
Signed-off-by: Ravi Shankar Jonnalagadda <ravis.opensrc@micron.com>
Signed-off-by: Bijan Tabatabai <bijantabatab@micron.com>
Reviewed-by: SeongJae Park <sj@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>