[docs] Add missing steps to Testing Presubmit Engine PRs

Most framework tests perform a `flutter precache` or rely on the web SDK
in some form. The Web SDK is not build in presubmit by default. As such,
developers will need to manually comment out the `runIf` conditional
block that prevents the web SDK from being built during presubmit, then
later hope they uncomment it before landing their patch.

Author: cbracken

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

@@ -4,12 +4,12 @@ This documentation describes how to run flutter/flutter presubmit checks on flut
 
 # Overview
 
+1. Edit `.ci.yaml`, comment out the `runIf:` block in `linux_web_engine` to ensure Flutter Web artifacts build (otherwise most framework tests will fail during precache).
 1. Wait for all presubmit checks on your flutter/engine PR to be green.
-2. Determine the commit hash for your flutter/engine PR.
-3. Create and upload a flutter/flutter PR, (OR run tests locally).
-4. Wait for flutter/flutter presubmits/tests to run ☕.
-
-Step (1) is the usual flutter/engine workflow.
+1. Determine the commit hash for your flutter/engine PR.
+1. Create and upload a flutter/flutter PR, (OR run tests locally).
+1. Wait for flutter/flutter presubmits/tests to run ☕.
+1. Edit `.ci.yaml` to uncomment the `runIf` block you commented out above.
 
 # 2. The commit hash