[Impeller] Update docs that specify how to select the Impeller backend.

impeller/README.md

@@ -197,7 +197,7 @@ Flutter enables Impeller by **default** on iOS.
 To **disable** Impeller on iOS, update your `Info.plist` file to add the following
 under the top-level `<dict>` tag:
 
-```
+```xml
   <key>FLTEnableImpeller</key>
   <false/>
 ```
@@ -207,18 +207,35 @@ under the top-level `<dict>` tag:
 Impeller is in preview on Android.
 
 To your `AndroidManifest.xml` file, add under the `<application>` tag:
-```
+
+```xml
   <meta-data
     android:name="io.flutter.embedding.android.EnableImpeller"
     android:value="true" />
 ```
 
+Impeller will use Vulkan on Android by default when opted into. Where Vulkan
+is unavailable, Impeller will fallback to Skia. However, Impellers OpenGL backend
+is well under construction. To try that with your application, add the following
+under the `<application>` tag:
+
+> [!Warning]
+> Selecting the Impeller backend this way will only work in `debug` and `profile`
+> runtime modes.
+
+```xml
+  <meta-data
+    android:name="io.flutter.embedding.android.ImpellerBackend"
+    android:value="opengles" />
+```
+
 ### macOS Desktop
 
 Impeller is in preview on macOS Desktop.
 
 To your `Info.plist` file, add under the top-level `<dict>` tag:
-```
+
+```xml
   <key>FLTEnableImpeller</key>
   <true/>
 ```