cmd/go: clarify behavior with no arguments in 'go help mod download'

'go mod download' is equivalent to 'go mod download all'.

Fixes #38031

Change-Id: I7aec7e5a1370a3e248eba6daad9a75ec21f33a83
Reviewed-on: https://go-review.googlesource.com/c/go/+/225201


Run-TryBot: Jay Conrod <jayconrod@google.com>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>