Update docs for checkout with 'fetch' tool. Bug: angleproject:7018 Change-Id: I3e7e905ce536353bb07e260302bfba07d4a4a619 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3465504 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>