cgroup: document cgroup_no_v1=

Add cgroup_no_v1= to kernel-parameters.txt, and a small blurb to
cgroup-v2.txt section about transitioning from cgroup to cgroup2.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Tejun Heo <tj@kernel.org>
diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index cfb2c0f..0d962a1 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -608,6 +608,11 @@
 			cut the overhead, others just disable the usage. So
 			only cgroup_disable=memory is actually worthy}
 
+	cgroup_no_v1=	[KNL] Disable one, multiple, all cgroup controllers in v1
+			Format: { controller[,controller...] | "all" }
+			Like cgroup_disable, but only applies to cgroup v1;
+			the blacklisted controllers remain available in cgroup2.
+
 	cgroup.memory=	[KNL] Pass options to the cgroup memory controller.
 			Format: <string>
 			nosocket -- Disable socket memory accounting.