Updated Stateless and Stateful widget docstrings (#95407)

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

@@ -438,6 +438,14 @@ abstract class Widget extends DiagnosticableTree {
 ///  * Use `const` widgets where possible, and provide a `const` constructor for
 ///    the widget so that users of the widget can also do so.
 ///
+///  * When trying to create a reusable piece of UI, prefer using a widget
+///    rather than a helper method. For example, if there was a function used to
+///    build a widget, a [State.setState] call would require Flutter to entirely
+///    rebuild the returned wrapping widget. If a [Widget] was used instead,
+///    Flutter would be able to efficiently re-render only those parts that
+///    really need to be updated. Even better, if the created widget is `const`,
+///    Flutter would short-circuit most of the rebuild work.
+///
 ///  * Consider refactoring the stateless widget into a stateful widget so that
 ///    it can use some of the techniques described at [StatefulWidget], such as
 ///    caching common parts of subtrees and using [GlobalKey]s when changing the
@@ -646,11 +654,20 @@ abstract class StatelessWidget extends Widget {
 ///    efficient for a widget to be re-used than for a new (but
 ///    identically-configured) widget to be created. Factoring out the stateful
 ///    part into a widget that takes a child argument is a common way of doing
-///    this.
+///    this. Another caching strategy consists of assigning a widget to a
+///    `final` state variable which can be used in the build method.
 ///
 ///  * Use `const` widgets where possible. (This is equivalent to caching a
 ///    widget and re-using it.)
 ///
+///  * When trying to create a reusable piece of UI, prefer using a widget
+///    rather than a helper method. For example, if there was a function used to
+///    build a widget, a [State.setState] call would require Flutter to entirely
+///    rebuild the returned wrapping widget. If a [Widget] was used instead,
+///    Flutter would be able to efficiently re-render only those parts that
+///    really need to be updated. Even better, if the created widget is `const`,
+///    Flutter would short-circuit most of the rebuild work.
+///
 ///  * Avoid changing the depth of any created subtrees or changing the type of
 ///    any widgets in the subtree. For example, rather than returning either the
 ///    child or the child wrapped in an [IgnorePointer], always wrap the child