memcg: rename cgroup_event to mem_cgroup_event

cgroup_event is only available in memcg now.  Let's brand it that way.
While at it, add a comment encouraging deprecation of the feature and
remove the respective section from cgroup documentation.

This patch is cosmetic.

v3: Typo update as per Li Zefan.

v2: Index in cgroups.txt updated accordingly as suggested by Li Zefan.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Acked-by: Michal Hocko <mhocko@suse.cz>