mojo_run: support parallel Android runs of mojo_run.

mojo/devtools/common/docs/mojo_run.md

 # mojo_run
 
 `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 meant to be run embedded in a **window manager**. To run
 these, you can pass the app url using the `--embed` flag. This will run the
 window manager and pass the given url to it:
 
 ```sh
 mojo_run --embed APP_URL [--android]
 ```
 
 By default, `mojo_run` uses mojo:kiosk_wm as the window manager. You can pass a
 different window manager url using the `--window-manager` flag to override this.
+
+## Running on multiple Android devices
+
+Two or more instances of `mojo_run` can simultaneously run on separate Android
+devices. For that, run in individual shells:
+
+```sh
+mojo_run APP_URL --android --target-device DEVICE_ID --free-host-ports
+```
+
+```sh
+mojo_run APP_URL --android --target-device ANOTHER_DEVICE_ID --free-host-ports
+```
+
+`--free-host-ports` makes `mojo_run` spawn the development servers on
+system-allocated ports on the server (so that multiple instances can run in
+parallel) while still forwarding them to fixed ports on the device (so that
+caching still works). This breaks the remote workflow over `adb_remote_setup`.
+
+DEVICE_ID can be obtained from `adb devices`.