Add docs to explain cursor drawing behavior. (#24135)

packages/flutter/lib/src/widgets/editable_text.dart

@@ -417,6 +417,12 @@ class EditableText extends StatefulWidget {
   /// How thick the cursor will be.
   ///
   /// Defaults to 2.0
+  ///
+  /// The cursor will draw under the text. The cursor width will extend
+  /// to the right of the boundary between characters for left-to-right text
+  /// and to the left for right-to-left text. This corresponds to extending
+  /// downstream relative to the selected position. Negative values may be used
+  /// to reverse this behavior.
   /// {@endtemplate}
   final double cursorWidth;