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.orgSigned-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>

Documentation/mm/damon/design.rst

@@ -163,9 +163,10 @@ assumption (pages in a region have the same access frequencies) is kept, only
 one page in the region is required to be checked.  Thus, for each ``sampling
 interval``, DAMON randomly picks one page in each region, waits for one
 ``sampling interval``, checks whether the page is accessed meanwhile, and
-increases the access frequency of the region if so.  Therefore, the monitoring
-overhead is controllable by setting the number of regions.  DAMON allows users
-to set the minimum and the maximum number of regions for the trade-off.
+increases the access frequency counter of the region if so.  The counter is
+called ``nr_regions`` of the region.  Therefore, the monitoring overhead is
+controllable by setting the number of regions.  DAMON allows users to set the
+minimum and the maximum number of regions for the trade-off.
 
 This scheme, however, cannot preserve the quality of the output if the
 assumption is not guaranteed.