[docs/android_emulator] Highlight need of remount for writable system

Change-Id: If6708d6312bb4bf48e1d53e9245ae88a92ed1dbc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3501520
Commit-Queue: Victor Vianna <victorvianna@google.com>
Auto-Submit: Victor Vianna <victorvianna@google.com>
Reviewed-by: Michael Thiessen <mthiesse@chromium.org>
Commit-Queue: Michael Thiessen <mthiesse@chromium.org>
Cr-Commit-Position: refs/heads/main@{#977666}

docs/android_emulator.md

@@ -172,7 +172,12 @@ To manage emulator lifetime independently, use `tools/android/avd/avd.py`.
 
  * `--writable-system`
 
-    Makes system & vendor image writable after adb remount.
+    Makes system & vendor image writable. It's necessary to run
+    ```
+    adb root
+    adb remount
+    ```
+    after the emulator starts.
 
  * `--debug-tags`