cmd/go: provide a more helpful suggestion for "go vet -?"

For the command

	go vet -?

the output was,

	usage: go vet [-n] [-x] [-vettool prog] [build flags] [vet flags] [packages]
	Run 'go help vet' for details.
	Run 'go tool vet -help' for the vet tool's flags.

but "go help vet" is perfunctory at best. (That's another issue I'm
working on—see https://go-review.googlesource.com/c/tools/+/291909—
but vendoring is required to sort that out.) Add another line and rewrite
a bit to make it actually helpful:

	usage: go vet [-n] [-x] [-vettool prog] [build flags] [vet flags] [packages]
	Run 'go help vet' for details.
	Run 'go tool vet help' for a full list of flags and analyzers.
	Run 'go tool vet -help' for an overview.

Change-Id: I9d8580f0573321a57d55875ac3185988ce3eaf64
Reviewed-on: https://go-review.googlesource.com/c/go/+/291929


Trust: Rob Pike <r@golang.org>
Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>