[TextSelectionTheme] improve docs (#75967)

packages/flutter/lib/src/material/text_selection_theme.dart

@@ -44,6 +44,10 @@ class TextSelectionThemeData with Diagnosticable {
   ///
   /// Selection handles are used to indicate the bounds of the selected text,
   /// or as a handle to drag the cursor to a new location in the text.
+  ///
+  /// On iOS [TextField] and [SelectableText] cannot access [selectionHandleColor].
+  /// To set the [selectionHandleColor] on iOS, you can change the
+  /// [CupertinoThemeData.primaryColor] in [ThemeData.cupertinoOverrideTheme].
   final Color? selectionHandleColor;
 
   /// Creates a copy of this object with the given fields replaced with the