os: document that Remove removes only empty directories

Fixes #26507

Change-Id: I967e4f897ca891c70ca8bb0e66b984530c240815
Reviewed-on: https://go-review.googlesource.com/127835Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

src/os/file_unix.go

@@ -296,7 +296,7 @@ func Truncate(name string, size int64) error {
 	return nil
 }
 
-// Remove removes the named file or directory.
+// Remove removes the named file or (empty) directory.
 // If there is an error, it will be of type *PathError.
 func Remove(name string) error {
 	// System call interface forces us to know