trivial: fix where cgroup documentation is not correctly referred to

cgroup documentation was moved to Documentation/cgroups/. There are some
places that still refer to Documentation/controllers/,
Documentation/cgroups.txt and Documentation/cpusets.txt. Fix those.

Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@holoscopio.com>
Reviewed-by: Li Zefan <lizf@cn.fujitsu.com>
Acked-by: Paul Menage <menage@google.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

Documentation/cgroups/00-INDEX

@@ -0,0 +1,18 @@
+00-INDEX
+	- this file
+cgroups.txt
+	- Control Groups definition, implementation details, examples and API.
+cpuacct.txt
+	- CPU Accounting Controller; account CPU usage for groups of tasks.
+cpusets.txt
+	- documents the cpusets feature; assign CPUs and Mem to a set of tasks.
+devices.txt
+	- Device Whitelist Controller; description, interface and security.
+freezer-subsystem.txt
+	- checkpointing; rationale to not use signals, interface.
+memcg_test.txt
+	- Memory Resource Controller; implementation details.
+memory.txt
+	- Memory Resource Controller; design, accounting, interface, testing.
+resource_counter.txt
+	- Resource Counter API.