os/exec: Document the fact that Cmd cannot be reused.

Update #10305

Change-Id: Iea04758bc200038a1c64457a68100dcdd7f75212
Reviewed-on: https://go-review.googlesource.com/9440Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/os/exec/exec.go

@@ -32,6 +32,9 @@ func (e *Error) Error() string {
 }
 
 // Cmd represents an external command being prepared or run.
+//
+// A Cmd cannot be reused after calling its Run, Output or CombinedOutput
+// methods.
 type Cmd struct {
 	// Path is the path of the command to run.
 	//