cgroup: update outdated cgroup.procs documentation

576dd46450 ("cgroup: drop the matching uid requirement on migration
for cgroup v2") dropped the uid match requirement from "cgroup.procs"
perm check but forgot to update the matching entry in the
documentation.  Update it.

Signed-off-by: Tejun Heo <tj@kernel.org>

Documentation/cgroup-v2.txt

@@ -658,9 +658,6 @@ All cgroup core files are prefixed with "cgroup."
 	the PID to the cgroup.  The writer should match all of the
 	following conditions.
 
-	- Its euid is either root or must match either uid or suid of
-          the target process.
-
 	- It must have write access to the "cgroup.procs" file.
 
 	- It must have write access to the "cgroup.procs" file of the