[release-branch.go1.4] cmd/go: document that -run isn't implemented

I am an idiot but the failure to implement this means we can decide
exactly what its design should be for 1.5

Change-Id: Ie2b025fcd899d306ddeddd09d1d0e8f9a99ab7a8
Reviewed-on: https://go-review.googlesource.com/4291


Reviewed-by: Minux Ma <minux@golang.org>
(cherry picked from commit 1e5d8bb5)
Reviewed-on: https://go-review.googlesource.com/4998


Reviewed-by: David Symonds <dsymonds@golang.org>