[Impeller] Add and reorder FAQ entries. (#39106)

* [Impeller] Add and reorder FAQ entries.

* Update impeller/docs/faq.md

Co-authored-by: Loïc Sharma <[email protected]>

Co-authored-by: Loïc Sharma <[email protected]>

Author: chinmaygarde

impeller/docs/faq.md

@@ -1,15 +1,53 @@
 # Frequently Asked Questions
 
-* How do you run `impeller_unittests` with Playgrounds enabled?
-  * Playgrounds in the `impeller_unittests` harness can be enabled in one of
-    three ways:
-    * Edit `gn args` directly and add `impeller_enable_playground = true`.
-    * Add the `--enable-impeller-playground` flag to your `./flutter/tools/gn`
-      invocation.
-    * Set the `FLUTTER_IMPELLER_ENABLE_PLAYGROUND` to `1` before invoking
-      `./flutter/tools/gn`. Only do this if you frequently work with Playgrounds
-      and don't want to have to set the flags manually. Also, it would be a bad
-      idea to set this environment variable on CI.
+* How do I enable Impeller to try it out myself?
+  * See the instructions in the README on how to [try Impeller in
+    Flutter](https://github.com/flutter/engine/tree/main/impeller#try-impeller-in-flutter).
+  * Support on some platforms is further along than on others. The current
+    priority for the team is to support iOS, Android, Desktops, and Embedder API
+    users (in that rough order).
+* I am running into issues when Impeller is enabled, how do I report them?
+  * Like any other Flutter issue, you can report them on the [GitHub issue
+    tracker](https://github.com/flutter/flutter/issues/new/choose).
+  * Please explicitly mention that this is an Impeller specific regression. You
+    can quickly swap between the Impeller and Skia backends using the command
+    line flag flag detailed in [section in the README on how to try
+    Impeller](https://github.com/flutter/engine/tree/main/impeller#try-impeller-in-flutter).
+  * Reduced test cases are the most useful.
+  * Please also report any performance regressions.
+* What does it mean for an Impeller platform to be "in preview". How long will
+  be the preview last?
+  * The team is focused on getting one platform right at time. This includes
+    ensuring all fidelity issues are fixed, performance issues addressed, and
+    compatibility with plugins guaranteed.
+  * When the team believes that the majority of Flutter applications will
+    benefit from Impeller on a specific platform, the backend will be declared
+    to be in preview.
+  * During the preview, Flutter developers will need to opt in to using
+    Impeller.
+  * The top priority of the team will be to address issues reported by
+    developers opting into the preview. The team wants the preview phase for a
+    platform to be as short as possible.
+  * Once major issues reported by platforms in preview become manageable, the
+    preview ends and Impeller becomes the default rendering backend.
+  * Besides working on fixing issues reported on platforms in preview, and
+    working on supporting additional platforms, the team is also undertaking a
+    high-touch exercise of migrating large existing Flutter applications to use
+    Impeller. The team will find and fix any issues it encounters during this
+    exercise.
+  * The length of the preview will depend on the number and nature of the issues
+    filed by developers and discovered by the team.
+  * Even once the preview ends, the developer can opt into the legacy rendering
+    backend for a short period of time. The legacy backend will be removed after
+    this period.
+* What can I expect when I opt in to using Impeller?
+  * A high level overview of the status of project is [present on the
+    wiki](https://github.com/flutter/flutter/wiki/Impeller#status).
+  * All Impeller related work items are tracked on a [project specific dashboard
+    on GitHub](https://github.com/orgs/flutter/projects/21).
+  * The team tracks known platform specific issues in their own milestones:
+    * [iOS](https://github.com/flutter/flutter/milestone/77)
+    * [Android](https://github.com/flutter/flutter/milestone/76)
 * Does Impeller use Skia for rendering?
   * No. Impeller has no direct dependencies on Skia.
   * When running with Impeller, Flutter does not create a Skia graphics context.
@@ -45,8 +83,9 @@
     priority at this time for the small team working on Impeller.
   * We are aware that these priorities might change in the future. There have
     been sanity checks to ensure that the Impeller API can be ported to WASM and
-    also that Impeller shaders can be [compiled to WGSL](https://github.com/chinmaygarde/wgsl_sandbox)
-    for eventual WebGPU support.
+    also that Impeller shaders can be [compiled to
+    WGSL](https://github.com/chinmaygarde/wgsl_sandbox) for eventual WebGPU
+    support.
 * How will Impeller affect the way in which Flutter applications are created and
   packaged?
   * It won't.
@@ -59,6 +98,13 @@
     includes all precompiled shaders.
   * Impeller is compiled into the Flutter engine. It is currently behind a flag
     as development progresses.
-* How do I enable Impeller to try it out myself?
-  * See the instructions in the README on how to [try Impeller in
-    Flutter](https://github.com/flutter/engine/tree/main/impeller#try-impeller-in-flutter).
+* How do you run `impeller_unittests` with Playgrounds enabled?
+  * Playgrounds in the `impeller_unittests` harness can be enabled in one of
+    three ways:
+    * Edit `gn args` directly and add `impeller_enable_playground = true`.
+    * Add the `--enable-impeller-playground` flag to your `./flutter/tools/gn`
+      invocation.
+    * Set the `FLUTTER_IMPELLER_ENABLE_PLAYGROUND` to `1` before invoking
+      `./flutter/tools/gn`. Only do this if you frequently work with Playgrounds
+      and don't want to have to set the flags manually. Also, it would be a bad
+      idea to set this environment variable on CI.