docs: Rework "Making changes to the `flutter` tool" to include `flutter-dev` (#157480)

Follow-up to https://github.com/flutter/flutter/pull/153599. This updates the tool readme, suggesting `bin/flutter-dev` as the primary way to run the flutter tool locally from source.

<details>

<summary> Pre-launch checklist </summary> 

</details>

docs/tool/README.md

diff --git a/docs/tool/README.md b/docs/tool/README.md
index ed4a23f..5f530ad 100644
--- a/docs/tool/README.md
+++ b/docs/tool/README.md
@@ -53,10 +53,12 @@
 
 ## Making changes to the `flutter` tool
 
-If you want to alter and re-test the tool's behavior itself, locally commit your tool change
-in git and the tool will be rebuilt from Dart sources in `packages/flutter_tools` the next
-time you run `flutter`. Alternatively, delete the `bin/cache/flutter_tools.snapshot` file.
-Doing so will force a rebuild of the tool from your local sources the next time you run `flutter`.
+You can run the tool from source by running `bin/flutter-dev`.
+
+Alternatively, delete the `bin/cache/flutter_tools.snapshot` file or locally commit
+your change in git and then run `flutter` again. This will rebuild the tool
+from local source.
+
 This step is not required if you are launching `flutter_tools.dart` (either by running or testing) from an IDE.
 
 The `flutter_tools` tests run inside the Dart command line VM rather than in the
@@ -78,7 +80,7 @@
 
 The pre-built flutter tool runs in release mode with the observatory off by default.
 To enable debugging mode and the observatory on the `flutter` tool, uncomment the
-`FLUTTER_TOOL_ARGS` line in the `bin/flutter` shell script.
+`FLUTTER_TOOL_ARGS` line in the `bin/flutter` (or `bin/flutter-dev`) shell script.
 
 ## Debugging the `flutter` command-line tool in VS Code