Documentation: memcg: future proof hierarchical statistics documentation

The hierarchical versions of per-memcg counters in memory.stat are all
calculated the same way and are all named total_<counter>.

Documenting the pattern is easier for maintenance than listing each
counter twice.

Signed-off-by: Johannes Weiner <[e-mail]>
Acked-by: Michal Hocko <[e-mail]>
Acked-by: KOSAKI Motohiro <[e-mail]>
Acked-by: Ying Han <[e-mail]>
Randy Dunlap <[e-mail]>
Acked-by: KAMEZAWA Hiroyuki <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

Documentation/cgroups/memory.txt

···
 hierarchical_memsw_limit - # of bytes of memory+swap limit with regard to
 			hierarchy under which memory cgroup is.
 
-total_cache		- sum of all children's "cache"
-total_rss		- sum of all children's "rss"
-total_mapped_file	- sum of all children's "cache"
-total_pgpgin		- sum of all children's "pgpgin"
-total_pgpgout		- sum of all children's "pgpgout"
-total_swap		- sum of all children's "swap"
-total_inactive_anon	- sum of all children's "inactive_anon"
-total_active_anon	- sum of all children's "active_anon"
-total_inactive_file	- sum of all children's "inactive_file"
-total_active_file	- sum of all children's "active_file"
-total_unevictable	- sum of all children's "unevictable"
+total_<counter>		- # hierarchical version of <counter>, which in
+			addition to the cgroup's own value includes the
+			sum of all hierarchical children's values of
+			<counter>, i.e. total_cache
 
 # The following additional stats are dependent on CONFIG_DEBUG_VM.