Make the docs for --cgroup-parent better

Signed-off-by: Marianna <mtesselh@gmail.com>

docs/sources/reference/commandline/cli.md

@@ -848,8 +848,10 @@ you refer to it on the command line.
 > children) for security reasons, and to ensure repeatable builds on remote
 > Docker hosts. This is also the reason why `ADD ../file` will not work.
 
-`docker build` has a `--cgroup-parent` option that causes the containers used
-in the build to be run with this option.
+When `docker build` is run with the `--cgroup-parent` option the containers used
+in the build will be run with the [corresponding `docker run`
+flag](/reference/run/#specifying-custom-cgroups). 
+
 
 ## commit
 

docs/sources/reference/run.md

@@ -465,6 +465,13 @@ Note:
 
 You would have to write policy defining a `svirt_apache_t` type.
 
+## Specifying custom cgroups
+
+Using the `--cgroup-parent` flag, you can pass a specific cgroup to run a
+container in. This allows you to create and manage cgroups on their own. You can
+define custom resources for those cgroups and put containers under a common
+parent group.
+
 ## Runtime constraints on resources
 
 The operator can also adjust the performance parameters of the