Docs/mm/damon/design: explicitly introduce ``nr_accesses``

The design document is explaining about the access tracking mechanism
and the access rate counter (nr_accesses), but not directly mentions the
name.  Add a sentence for making it clear.

Link: https://lkml.kernel.org/r/20230907022929.91361-5-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>