runtime-config-linux: Clarify resources vs. cgroupsPath

This should help clarify the cgroupsPath setting added in opencontainers#137, which
was the subject of some confusion in opencontainers/runc#397.  Issues
I'm trying to clarify here:

* If you specify a cgroupsPath, is the container added to that path or
  a sub-cgroup underneath it [1]?  (This commit rules in favor of
  "added to that path")

* If you specify a cgroupsPath, can the runtime modify that cgroup
  [2]?  (This commit rules "yes, if 'resources' is specified",
  following [3] and the Go comment from opencontainers#137 [4]).

To help make the distinctions clearer, I've added a facet list to help
folks think about the difference between cgroup creation, process
assignment, and resource configuration.  cgroupsPath is just about
cgroup creation and process assignment.  'resources' is just about
resource configuration.  I've listed out Mrunal's first three cases
[3] to be even clearer.  I stayed away from the "neither are set"
case, since I covered that fairly directly in opencontainers#237, which that was
punted back to the list [5] and has seen no further interest.  So I'm
not clear on what the intended semantics are there, although Mrunal's
wording in [4] seems to agree with the proposal in opencontainers#237.

[1]: opencontainers/runc#397 (comment)
[2]: opencontainers/runc#397 (comment)
[3]: opencontainers/runc#397 (comment)
[4]: opencontainers@429f936#diff-34c30be66233f08b447fb608ea0e66bbR30
[5]: https://groups.google.com/a/opencontainers.org/d/msg/dev/qWHoKs8Fsrk/c9mv6qXtDAAJ
     Message-ID: <20151029194427.GA30073@odin.tremily.us>

Signed-off-by: W. Trevor King <wking@tremily.us>

runtime-config-linux.md

@@ -144,6 +144,12 @@ Also known as cgroups, they are used to restrict resource usage for a container
 cgroups provide controls to restrict cpu, memory, IO, pids and network for the container.
 For more information, see the [kernel cgroups documentation](https://www.kernel.org/doc/Documentation/cgroups/cgroups.txt).
 
+There are a few facets to this:
+
+* Managing the existence of cgroups within the hierarchy (`cgroupsPath`).
+* Assigning processes to cgroups.
+* Configuring cgroups to apply resource limits (`resources`).
+
 The path to the cgroups can be specified in the Spec via `cgroupsPath`.
 `cgroupsPath` is expected to be relative to the cgroups mount point.
 If not specified, cgroups will be created under '/'.
@@ -156,11 +162,13 @@ The cgroups will be created if they don't exist.
    "cgroupsPath": "/myRuntime/myContainer"
 ```
 
-`cgroupsPath` can be used to either control the cgroups hierarchy for containers or to run a new process in an existing container.
+The container process will be added to the cgroups at `cgroupsPath`.
 
 You can configure a container's cgroups via the `resources` field of the Linux configuration.
 Do not specify `resources` unless limits have to be updated.
-For example, to run a new process in an existing container without updating limits, `resources` need not be specified.
+For example, to run a new process in an existing container without updating limits, specify `cgroupsPath` but not `resources`.
+To run a new process in an existing container and update limits, specify both `cgroupsPath` and `resources`.
+To run a new process in a new container with explicit resource limits, specify `resources` and optionally set `cgroupsPath` to a cgroup that does not yet exist.
 
 #### Disable out-of-memory killer