cgroup, doc: make cgroup_disable doc more accurate

In doc, it said that 'Currently supported controllers - "memory"',
but actually we can use cgroup_disable=cpu,cpuset and all other
controllers, so this is confusing for cgroup users without much
cgroup knowledge. We need to make it clear.

[some comments copied from Paul Menage's original patch 8bab8dded]

Signed-off-by: Qiang Huang <h.huangqiang@huawei.com>
Acked-by: Rob Landley <rob@landley.net>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

Documentation/kernel-parameters.txt

@@ -511,7 +511,14 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 
 	cgroup_disable= [KNL] Disable a particular controller
 			Format: {name of the controller(s) to disable}
-				{Currently supported controllers - "memory"}
+			The effects of cgroup_disable=foo are:
+			- foo isn't auto-mounted if you mount all cgroups in
+			  a single hierarchy
+			- foo isn't visible as an individually mountable
+			  subsystem
+			{Currently only "memory" controller deal with this and
+			cut the overhead, others just disable the usage. So
+			only cgroup_disable=memory is actually worthy}
 
 	checkreqprot	[SELINUX] Set initial checkreqprot flag value.
 			Format: { "0" | "1" }