The new flag -args stops flag processing, leaving the rest of the command line
to be passed to the underlying test binary verbatim. Thus, both of these pass
a literal -v -n on the test binary command line, without putting the go command
into verbose mode or disabling execution of commands:

	go test . -args -v -n
	go test -args -v -n

Also try to make the documentation a bit clearer.

Fixes #7221.
Fixes #12177.

Change-Id: Ief9e830a6fbb9475d96011716a86e2524a35eceb
Reviewed-on: https://go-review.googlesource.com/17775Reviewed-by: Rob Pike <r@golang.org>