mojo_run: support parallel Android runs of mojo_run.

mojo/devtools/common/docs/mojo_run.md

Index: mojo/devtools/common/docs/mojo_run.md
diff --git a/mojo/devtools/common/docs/mojo_run.md b/mojo/devtools/common/docs/mojo_run.md
index 1b96f6763b0150835c8722d208449ff76fd90da5..0675c9cf824c67ad71bac16cc06b8f939b30db65 100644
--- a/mojo/devtools/common/docs/mojo_run.md
+++ b/mojo/devtools/common/docs/mojo_run.md
@@ -26,3 +26,23 @@ 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`.