[AW] Update device-setup doc to callout not to use Google Play images

Add a sentence to callout not to use Google Play images on emulators.
Also add a sentence at the top of the document that `user` builds
aren't supported.

Fixed: 1217796
Test: Check rendering on gitiles
Change-Id: I3f1b08f0367051a3a351b88761130f0ce9f58da4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2949467
Reviewed-by: Nate Fischer <ntfschr@chromium.org>
Commit-Queue: Hazem Ashmawy <hazems@chromium.org>
Cr-Commit-Position: refs/heads/master@{#890825}

android_webview/docs/device-setup.md

@@ -5,7 +5,7 @@
 As WebView is an Android system component (rather than just an app), WebView
 imposes additional requirements on the developer workflow. In particular,
 WebView requires a physical device or emulator with a `userdebug` or `eng`
-Android image.
+Android image. WebView doesn't support development on `user` builds.
 
 To build WebView, or [run WebView's automated tests](./test-instructions.md),
 you'll need to set up either an emulator or a physical device.
@@ -37,7 +37,9 @@ easier to setup.
 You can generally follow chromium's [Android
 emulator](/docs/android_emulator.md) instructions. You should choose a **Google
 APIs** image. The AOSP-based image will also work, but imposes additional
-developer hurdles.
+developer hurdles. Note that you shouldn't use a **Google Play** image for
+development purposes because they are `user` builds, see [Why won't a user
+image work](#why-won_t-a-user-image-work) below.
 
 
 ## Physical device
@@ -47,7 +49,7 @@ developer hurdles.
 Googlers can consult internal instructions
 [here](http://go/clank-webview/device_setup.md).
 
-External contributors can flash a prebuilt userdebug image (based off
+External contributors can flash a prebuilt `userdebug` image (based off
 aosp-master) onto a Pixel device with [Android Flash
 Tool](https://flash.android.com/welcome?continue=%2Fcustom). This requires a
 browser capable of WebUSB (we recommend the latest Google Chrome stable