os/exec: document how Command fills in Cmd.Args

Fixes #17536

Change-Id: Ica8c3d696848822ac65b7931455b1fd94809bfe8
Reviewed-on: https://go-review.googlesource.com/31710Reviewed-by: Russ Cox <rsc@golang.org>

src/os/exec/exec.go

@@ -120,12 +120,13 @@ type Cmd struct {
 // It sets only the Path and Args in the returned structure.
 //
 // If name contains no path separators, Command uses LookPath to
-// resolve the path to a complete name if possible. Otherwise it uses
-// name directly.
+// resolve name to a complete path if possible. Otherwise it uses name
+// directly as Path.
 //
 // The returned Cmd's Args field is constructed from the command name
 // followed by the elements of arg, so arg should not include the
-// command name itself. For example, Command("echo", "hello")
+// command name itself. For example, Command("echo", "hello").
+// Args[0] is always name, not the possibly resolved Path.
 func Command(name string, arg ...string) *Cmd {
 	cmd := &Cmd{
 		Path: name,