os: add document for ErrInvalid

Fixes #12391.

Change-Id: I31211cd711c035151785c3083571594d15c7d93e
Reviewed-on: https://go-review.googlesource.com/16873Reviewed-by: Andrew Gerrand <adg@golang.org>

src/os/error.go

@@ -10,7 +10,7 @@ import (
 
 // Portable analogs of some common system call errors.
 var (
-	ErrInvalid    = errors.New("invalid argument")
+	ErrInvalid    = errors.New("invalid argument") // methods on File will return this error when the receiver is nil
 	ErrPermission = errors.New("permission denied")
 	ErrExist      = errors.New("file already exists")
 	ErrNotExist   = errors.New("file does not exist")