Add some details about the lifecycle of commands started with docker exec

Signed-off-by: Sven Dowideit <SvenDowideit@home.org.au>

docs/sources/reference/commandline/cli.md

@@ -666,7 +666,11 @@ You'll need two shells for this example.
 
 The `docker exec` command runs a new command in a running container.
 
-The `docker exec` command will typically be used after `docker run` or `docker start`.
+The command started using `docker exec` will only run while the container's primary
+process (`PID 1`) is running, and will not be restarted if the container is restarted.
+
+If the container is paused, then the `docker exec` command will wait until the
+container is unpaused, and then run.
 
 #### Examples