cmd/go: Update documentation of go run to match current behaviour.

Fixes #5511.

R=golang-dev, minux.ma
CC=golang-dev
https://golang.org/cl/9530043

src/cmd/go/doc.go

@@ -370,7 +370,6 @@ Usage:
 	go run [build flags] gofiles... [arguments...]
 
 Run compiles and runs the main package comprising the named Go source files.
-If no files are named, it compiles and runs all non-test Go source files.
 
 For more about build flags, see 'go help build'.
 

src/cmd/go/run.go

@@ -16,7 +16,6 @@ var cmdRun = &Command{
 	Short:     "compile and run Go program",
 	Long: `
 Run compiles and runs the main package comprising the named Go source files.
-If no files are named, it compiles and runs all non-test Go source files.
 
 For more about build flags, see 'go help build'.