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>

Documentation/cgroup-v2.txt

@@ -132,6 +132,12 @@ strongly discouraged for production use.  It is recommended to decide
 the hierarchies and controller associations before starting using the
 controllers after system boot.
 
+During transition to v2, system management software might still
+automount the v1 cgroup filesystem and so hijack all controllers
+during boot, before manual intervention is possible. To make testing
+and experimenting easier, the kernel parameter cgroup_no_v1= allows
+disabling controllers in v1 and make them always available in v2.
+
 
 2-2. Organizing Processes
 

Documentation/kernel-parameters.txt

@@ -608,6 +608,11 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 			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.