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>