runtime: Replace "process is stopped" with "process exits"

proc(5) describes the following state entries in proc/[pid]/stat [1]
(for modern kernels):

* R Running
* S Sleeping in an interruptible wait
* D Waiting in uninterruptible disk sleep
* Z Zombie
* T Stopped (on a signal)
* t Tracing stop
* X Dead

and ps(1) has a bit more context [2] (for modern kernels):

* D uninterruptible sleep (usually IO)
* R running or runnable (on run queue)
* S interruptible sleep (waiting for an event to complete)
* T stopped by job control signal
* t stopped by debugger during the tracing
* X dead (should never be seen)
* Z defunct ("zombie") process, terminated but not reaped by its
  parent

So I expect "stopped" to mean "process still exists but is paused,
e.g. by SIGSTOP".  And I expect "exited" to mean "process has finished
and is either a zombie or dead".

After this commit, 'git grep -i stop' only turns up poststop-hook
stuff, a reference in principles.md, a "stoppage" in LICENSE, and some
ChangeLog entries.

This also replaces "container's process" with "container process" to
match usage in the rest of the repository.  After this commit:

  $ git grep -i "container process" | wc -l
  13
  $ git grep -i "container's process" | wc -l
  1

This also removes a "them" leftover from a partial plural -> singular
reroll of be59415 (Split create and start, 2016-04-01, #384).

[1]: http://man7.org/linux/man-pages/man5/proc.5.html
[2]: http://man7.org/linux/man-pages/man1/ps.1.html

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

runtime.md

@@ -41,8 +41,8 @@ The lifecycle describes the timeline of events that happen from when a container
   However, some actions might only be available based on the current state of the container (e.g. only available while it is started).
 4. Runtime's `start` command is invoked with the unique identifier of the container.
    The runtime MUST run the user-specified code, as specified by [`process`](config.md#process-configuration).
-5. The container's process is stopped.
-   This MAY happen due to them erroring out, exiting, crashing or the runtime's `kill` operation being invoked.
+5. The container process exits.
+   This MAY happen due to erroring out, exiting, crashing or the runtime's `kill` operation being invoked.
 6. Runtime's `delete` command is invoked with the unique identifier of the container.
    The container MUST be destroyed by undoing the steps performed during create phase (step 2).