Doc fix for `DropdownButtonFormField.value` (#144427)

Fixes https://github.com/flutter/flutter/issues/144135

This PR is just a doc fix.

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

@@ -1555,6 +1555,8 @@ class _DropdownButtonState<T> extends State<DropdownButton<T>> with WidgetsBindi
 /// pass a [GlobalKey] to the constructor and use [GlobalKey.currentState] to
 /// save or reset the form field.
 ///
+/// The `value` parameter maps to [FormField.initialValue].
+///
 /// See also:
 ///
 ///  * [DropdownButton], which is the underlying text field without the [Form]

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

@@ -445,6 +445,9 @@ class FormField<T> extends StatefulWidget {
   final FormFieldBuilder<T> builder;
 
   /// An optional value to initialize the form field to, or null otherwise.
+  ///
+  /// This is called `value` in the [DropdownButtonFormField] constructor to be
+  /// consistent with [DropdownButton].
   final T? initialValue;
 
   /// Whether the form is able to receive user input.