glossary: Explicit PID-hierarchy example for container/runtime namespaces

On today's call, Brandon asked for clarification for the namespace
terminology [1].

[1]: http://ircbot.wl.linuxfoundation.org/meetings/opencontainers/2016/opencontainers.2016-05-04-17.00.log.html#l-58
     Although I was running my mouth at that point, so I didn't get
     his statement down in the logs directly ;).

Signed-off-by: W. Trevor King <[email protected]>

Author: wking

glossary.md

@@ -16,6 +16,15 @@ For example, namespaces, resource limits, and mounts are all part of the contain
 ## Container namespace
 
 On Linux, a leaf in the [namespace][namespaces.7] hierarchy in which the [configured process](config.md#process-configuration) executes.
+For example, with a PID namespace hierarchy like:
+
+| PID namespace | Parent namespace | Runtime process PID | Container process PID |
+| ------------- | -----------------| ------------------- | --------------------- |
+| *A*           | -                | 123                 | 678                   |
+| *B*           | *A*              | 45                  | 90                    |
+| *C*           | *B*              | 0                   | 1                     |
+
+where the container process was launched in a new [PID namespace][pid_namespaces.7], namespace *C* (in which the container process is executing) would be the container PID namespace.
 
 ## JSON
 
@@ -29,8 +38,12 @@ It reads the [configuration files](#configuration) from a [bundle](#bundle), use
 ## Runtime namespace
 
 On Linux, a leaf in the [namespace][namespaces.7] hierarchy from which the [runtime](#runtime) process is executed.
+With the hierarchy example in the [container namespace section](#container-namespace), namespace *B* (in which the runtime process handling [start][] is execting) would be the runtime PID namespace.
 New container namespaces will be created as children of the runtime namespaces.
 
+[start]: runtime.md#start
+
 [JSON]: http://json.org/
 [UTF-8]: http://www.unicode.org/versions/Unicode8.0.0/ch03.pdf
 [namespaces.7]: http://man7.org/linux/man-pages/man7/namespaces.7.html
+[pid_namespaces.7]: http://man7.org/linux/man-pages/man7/pid_namespaces.7.html