Documentation/cgroups/memory.txt: fix stat file documentation

pgpgout		- # of uncharging events to the memory cgroup. The uncharging

		event happens each time a page is unaccounted from the cgroup.

swap		- # of bytes of swap usage

inactive_anon	- # of bytes of anonymous memory and swap cache memory on

inactive_anon	- # of bytes of anonymous and swap cache memory on inactive

		LRU list.

active_anon	- # of bytes of anonymous and swap cache memory on active

		inactive LRU list.

		LRU list.

inactive_file	- # of bytes of file-backed memory on inactive LRU list.

active_file	- # of bytes of file-backed memory on active LRU list.

unevictable	- # of bytes of memory that cannot be reclaimed (mlocked etc).