Docs/admin-guide/mm/damon/usage: link design doc for details of kdamond and context

The explanation of kdamond and context is duplicated in the design and
the usage documents.  Replace that in the usage with links to those in
the design document.

Link: https://lkml.kernel.org/r/20230907022929.91361-8-sj@kernel.org


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