cmd/go: improve go vet documentation

- restore and rework cmd/vet/doc.go, which was clobbered during the vet-lite switch.
- document go vet -vettool=prog flag and how to run an alternative checker.
- make 'go vet -help' show how to list vet tool's flags.  Example:

	$ go vet -help
	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.

	$ go vet -vettool=~/bin/myvet -help
	usage: go vet [-n] [-x] [-vettool prog] [build flags] [vet flags] [packages]
	Run 'go help vet' for details.
	Run '~/bin/myvet help' for the vet tool's flags.

Updates #28840

Change-Id: Ieb79dfe29e1df074f865bc9a9d47b44199675d7d
Reviewed-on: https://go-review.googlesource.com/c/147018


Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>