Prior doc implied that "git clone" was one way to obtain a go1.4
bootstrap toochain, but it did not state this outright. Further,
the doc did not make it explicit in the "Fetch the repository"
section that one must necessarily "git clone" a second time in
the (presumed-to-be-uncommon) case where "git clone" had already
been perfomed in the "compiler binaries" section.

Updates #33402

Change-Id: Id70a6587b6ee09aca13559d63868b75cb07dff1e
Reviewed-on: https://go-review.googlesource.com/c/go/+/188900Reviewed-by: Ian Lance Taylor <iant@golang.org>