mm/damon/core: add more comments for nr_accesses

mm/damon/core: add more comments for nr_accesses

The comment on struct damon_region about nr_accesses field looks not
sufficient.  Many people actually used to ask what nr_accesses mean.
There is more detailed explanation of the mechanism on the comment for
struct damon_attrs, but it is also ambiguous, as it doesn't specify the
name of the counter for aggregating the access check results.  Make
those more detailed.

Link: https://lkml.kernel.org/r/20230907022929.91361-10-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>