doc: memcontrol: fix the obsolete content about force empty

 memory.soft_limit_in_bytes	 # set/show soft limit of memory usage

 memory.stat			 # show various statistics

 memory.use_hierarchy		 # set/show hierarchical account enabled

 memory.force_empty		 # trigger forced move charge to parent

 memory.force_empty		 # trigger forced page reclaim

 memory.pressure_level		 # set memory pressure notifications

 memory.swappiness		 # set/show swappiness parameter of vmscan

				 (See sysctl's vm.swappiness)

  the cgroup will be reclaimed and as many pages reclaimed as possible.

  The typical use case for this interface is before calling rmdir().

  Because rmdir() moves all pages to parent, some out-of-use page caches can be

  moved to the parent. If you want to avoid that, force_empty will be useful.

  Though rmdir() offlines memcg, but the memcg may still stay there due to

  charged file caches. Some out-of-use page caches may keep charged until

  memory pressure happens. If you want to avoid that, force_empty will be useful.

  Also, note that when memory.kmem.limit_in_bytes is set the charges due to

  kernel pages will still be seen. This is not considered a failure and the