is blocked by:

• SR-13612 utils/update-checkout --scheme main does not work

Issue Description:

Under docs/HowToGuides/GettingStarted.md, under Cloning the Project there is a mention of using

utils/update-checkout --branch mybranchname

to update the branch of all swift-project repositories, but `update-checkout` no longer seems to have this flag. According to help page, the branches flag seems to have been replaced with this more fine-grained flag:

--scheme BRANCH-SCHEME
                        Use branches from the specified branch-scheme. A
                        "branch-scheme" is a list of (repo, branch) pairs.

So perhaps the docs should be updated to reflect that and give an example.