update build doc string to advocate avoiding doing tasks other than b… (#58213)

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

@@ -471,6 +471,9 @@ class StreamBuilder<T> extends StreamBuilderBase<T, AsyncSnapshot<T>> {
        super(key: key, stream: stream);
 
   /// The build strategy currently used by this builder.
+  ///
+  /// This builder must only return a widget and should not have any side
+  /// effects as it may be called multiple times.
   final AsyncWidgetBuilder<T> builder;
 
   /// The data that will be used to create the initial snapshot.
@@ -686,6 +689,9 @@ class FutureBuilder<T> extends StatefulWidget {
   ///    the value to which the future completed. If it completed with an error,
   ///    [AsyncSnapshot.hasError] will be true and [AsyncSnapshot.error] will be
   ///    set to the error object.
+  ///
+  /// This builder must only return a widget and should not have any side
+  /// effects as it may be called multiple times.
   final AsyncWidgetBuilder<T> builder;
 
   /// The data that will be used to create the snapshots provided until a

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

@@ -662,9 +662,11 @@ abstract class StatelessWidget extends Widget {
 
   /// Describes the part of the user interface represented by this widget.
   ///
-  /// The framework calls this method when this widget is inserted into the
-  /// tree in a given [BuildContext] and when the dependencies of this widget
-  /// change (e.g., an [InheritedWidget] referenced by this widget changes).
+  /// The framework calls this method when this widget is inserted into the tree
+  /// in a given [BuildContext] and when the dependencies of this widget change
+  /// (e.g., an [InheritedWidget] referenced by this widget changes). This
+  /// method can potentially be called in every frames should not have any side
+  /// effects beyond building a widget.
   ///
   /// The framework replaces the subtree below this widget with the widget
   /// returned by this method, either by updating the existing subtree or by
@@ -1315,7 +1317,8 @@ abstract class State<T extends StatefulWidget> with Diagnosticable {
 
   /// Describes the part of the user interface represented by this widget.
   ///
-  /// The framework calls this method in a number of different situations:
+  /// The framework calls this method in a number of different situations. For
+  /// example:
   ///
   ///  * After calling [initState].
   ///  * After calling [didUpdateWidget].
@@ -1325,6 +1328,9 @@ abstract class State<T extends StatefulWidget> with Diagnosticable {
   ///  * After calling [deactivate] and then reinserting the [State] object into
   ///    the tree at another location.
   ///
+  /// This method can potentially be called in every frames and should not have
+  /// any side effects beyond building a widget.
+  ///
   /// The framework replaces the subtree below this widget with the widget
   /// returned by this method, either by updating the existing subtree or by
   /// removing the subtree and inflating a new subtree, depending on whether the