dart:io | Add documentation changes to Link, File and Directory delete methods.

sdk/lib/io/file.dart

Index: sdk/lib/io/file.dart
diff --git a/sdk/lib/io/file.dart b/sdk/lib/io/file.dart
index 85aa1b346599732abf16086dd56a90efd7bc0697..eff8778f8828bcb414dcb1224c72151acff6f856 100644
--- a/sdk/lib/io/file.dart
+++ b/sdk/lib/io/file.dart
@@ -76,12 +76,14 @@ abstract class File extends FileSystemEntity {
 
   /**
    * Delete the file. Returns a [:Future<File>:] that completes with
-   * the file when it has been deleted.
+   * the file when it has been deleted. Only a file or a link to a file
+   * can be deleted with this method, not a directory or a broken link.
    */
   Future<File> delete();
 
   /**
-   * Synchronously delete the file.
+   * Synchronously delete the file. Only a file or a link to a file
+   * can be deleted with this method, not a directory or a broken link.
    *
    * Throws a [FileIOException] if the operation fails.
    */