Update led docs (#167380)

Update led docs to include `led edit -pa is_fusion='true'`

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

dev/bots/README.md

@@ -70,7 +70,7 @@ The typical cycle for editing a recipe is:
    the existing expected output to match the new output. Verify completely new test
    cases by altering the `GenTests` method of the recipe. The recipe is required
    to have 100% test coverage.
-4. Run `led get-builder 'luci.flutter.staging:BUILDER_NAME' | led edit -pa git_ref='refs/pull/<PR number>/head' | led edit -pa git_url='https://github.com/flutter/<repo>' | led edit-recipe-bundle | led launch`, where `BUILDER_NAME` is the builder name (e.g. `Linux Engine`), and
+4. Run `led get-builder 'luci.flutter.staging:BUILDER_NAME' | led edit -pa git_ref='refs/pull/<PR number>/head' | led edit -pa git_url='https://github.com/flutter/<repo>' | led edit-recipe-bundle | led edit -pa is_fusion='true' | led launch`, where `BUILDER_NAME` is the builder name (e.g. `Linux Engine`), and
    `git_ref`/`git_url` is the ref/url of the intended changes to build.
    * If `led` fails, ensure that your `depot_tools` checkout is up to date.
 5. To submit a CL, you need a local branch first (`git checkout -b [some branch name]`).