Update stop.md
Updates the stop.md doc to mention that the stop signal can be changed, either with the Dockerfile or via `docker run --stop-signal`. This is a real gotcha if you're not familiar with this feature and build a container that extends a container that uses `STOPSIGNAL`.
@@ -19,7 +19,9 @@ Options:
## Description
The main process inside the container will receive `SIGTERM`, and after a grace
-period, `SIGKILL`.
+period, `SIGKILL`. The first signal can be changed with the `STOPSIGNAL`
+instruction in the container's Dockerfile, or the `--stop-signal` option to
+`docker run`.
## Examples
