Fix devtool documentation regarding app embedding

mojo/devtools/common/README.md

 # Devtools
 
 Unopinionated tools for **running**, **debugging** and **testing** Mojo apps.
 
 ## Install
 
 ```
 git clone https://github.com/domokit/devtools.git
 ```
 
@@ skipping 11 matching lines @@
 ports used by `mojo_run`.
 
 ### Runner
 
 `mojo_run` allows you to run a Mojo shell either on the host, or on an attached
 Android device.
 
 ```sh
 mojo_run APP_URL  # Run on the host.
 mojo_run APP_URL --android  # Run on Android device.
+mojo_run "APP_URL APP_ARGUMENTS"  # Run an app with startup arguments
 ```
 
 Unless running within a Mojo checkout, we need to indicate the path to the shell
 binary:
 
 ```sh
 mojo_run --shell-path path/to/shell/binary APP_URL
 ```
 
+Some applications are running embedded inside a window manager. To start such an
+app, you have to first start the window manager app, then have it embed the app
+you are interested in. It is done as follows using the default window manager:
+
+```sh
+mojo_run "mojo:window_manager APP_URL"
+```
+
+By default, `mojo_run` uses `mojo:kiosk_wm` as the default window manager. It
+can be changed using the `--window-manager` flag.
+
 #### Sky apps
 
 To run a [Sky](https://github.com/domokit/sky_engine) app, you need to build
 `sky_viewer.mojo` in a Sky checkout, and indicate the path to the binary using
 the `--map-url` parameter:
 
 ```sh
-mojo_run --map-url mojo:sky_viewer=/path/to/sky/viewer APP_URL
+mojo_run --map-url mojo:sky_viewer=/path/to/sky/viewer "mojo:window_manager APP_URL"
 ```
 
 If the app does not declare a shebang indicating that it needs to be run in
 `sky_viewer`, pass `--sky` to map `sky_viewer` as a default content handler for
 dart apps:
 
 ```sh
-mojo_run --map-url mojo:sky_viewer=/path/to/sky/viewer APP_URL --sky
+mojo_run --map-url mojo:sky_viewer=/path/to/sky/viewer "mojo:window_manager APP_URL" --sky
 ```
 
 Note that Sky apps will need the --use-osmesa flag to run
 over [chromoting](https://support.google.com/chrome/answer/1649523?hl=en):
 
 ### Debugger
 
 `mojo_debug` allows you to interactively inspect a running shell, collect
 performance traces and attach a gdb debugger.
 
@@ skipping 56 matching lines @@
 Linux and support for apptest frameworks. The executable scripts in devtools are
 based on this module. One can also choose to embed the functionality provided by
 **devtoolslib** in their own wrapper.
 
 ## Development
 
 The library is canonically developed [in the mojo
 repository](https://github.com/domokit/mojo/tree/master/mojo/devtools/common),
 https://github.com/domokit/devtools is a mirror allowing to consume it
 separately.