cmd/go: update go clean help message
Update the help message for the go clean command to include
the correct usage and flags for better clarity.
This change follows instructions by Ian on this thread <https://groups.google.com/g/golang-nuts/c/VENQ0fqLCSc/m/qO8EuawVBwAJ?pli=1>.
Change-Id: Ia509a38ee9ec7c31d384c3563535c5e3ccd9a9ce
GitHub-Last-Rev:
3048b2e4bd10a8d25da479f96c9d90ecb3c63082
GitHub-Pull-Request: golang/go#68135
Reviewed-on: https://go-review.googlesource.com/c/go/+/593639
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>
Commit-Queue: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>