os: add document for ErrInvalid

Fixes #12391.

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

diff --git a/src/os/error.go b/src/os/error.go
index 8810e693067b239a4254872fd6d60ac17f432471..e26ce279706933393e2c21208d783527bb04d609 100644 (file)
--- a/src/os/error.go
+++ b/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")