os: clarify that Close cancels pending I/O

Change-Id: I6be6818d951a999f916c2266a6753a5ce5144ee7
Reviewed-on: https://go-review.googlesource.com/122955Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

src/os/file_plan9.go

@@ -133,7 +133,8 @@ func openFileNolog(name string, flag int, perm FileMode) (*File, error) {
 }
 
 // Close closes the File, rendering it unusable for I/O.
-// It returns an error, if any.
+// On files that support SetDeadline, any pending I/O operations will
+// be canceled and return immediately with an error.
 func (f *File) Close() error {
 	if err := f.checkValid("close"); err != nil {
 		return err

src/os/file_unix.go

@@ -217,7 +217,8 @@ func openFileNolog(name string, flag int, perm FileMode) (*File, error) {
 }
 
 // Close closes the File, rendering it unusable for I/O.
-// It returns an error, if any.
+// On files that support SetDeadline, any pending I/O operations will
+// be canceled and return immediately with an error.
 func (f *File) Close() error {
 	if f == nil {
 		return ErrInvalid

src/os/file_windows.go

@@ -171,7 +171,8 @@ func openFileNolog(name string, flag int, perm FileMode) (*File, error) {
 }
 
 // Close closes the File, rendering it unusable for I/O.
-// It returns an error, if any.
+// On files that support SetDeadline, any pending I/O operations will
+// be canceled and return immediately with an error.
 func (file *File) Close() error {
 	if file == nil {
 		return ErrInvalid