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>

src/cmd/go/generate.go

@@ -106,6 +106,7 @@ The generator is run in the package's source directory.
 Go generate accepts one specific flag:
 
 	-run=""
+		TODO: This flag is unimplemented.
 		if non-empty, specifies a regular expression to
 		select directives whose command matches the expression.