docs: use test: all rather than editing .ci.yaml

docs/Testing-presubmit-Engine-PRs-with-the-Flutter-framework.md

@@ -4,16 +4,26 @@ This documentation describes how to run flutter/flutter presubmit checks on flut
 
 ## Overview
 
-1. Enable the web SDK presubmits.
+1. Create your engine pull request with the `test: all` label.
 2. Wait for all presubmit checks on your flutter/engine PR to be green.
 3. Determine the commit hash for your flutter/engine PR.
 4. Create and upload a flutter/flutter PR, (OR run tests locally).
 5. Wait for flutter/flutter presubmits/tests to run ☕.
-6. Edit `.ci.yaml` to uncomment the `runIf` block you commented out above.
 
-## 1. Edit .ci.yaml
+## 1. Create your engine pull request with the `test: all` label.
 
-Edit [`.ci.yaml`](https://github.com/flutter/engine/blob/main/.ci.yaml) to comment out the `runIf:` block in `linux_web_engine`. This will ensure Flutter Web artifacts build, otherwise most framework tests will fail during precache.
+When creating your PR, add the `test: all` label *before* submitting the PR to
+presubmit. This will ensure that all builds required for framework testing are
+triggered.
+
+If you sent out your PR without adding the `test: all` label, add it, then
+re-push your branch to re-trigger presubmits.
+
+By default, not all builds and tests are run in engine presubmits. When our CI
+is able to determine that certain shards are unaffected by a change, via a
+`runIf` clause in our `.ci.yaml`, for example, it will be skipped. Many
+framework tests, however, assume all build products are present and will trigger
+a `flutter precache`, which will fail with a 404 on missing build artifacts.
 
 ## 2. Wait