Minor doc changes (#52433)

Just some very minor doc changes: an adjustment of my comments in #51495, and a typo fix.

packages/flutter/lib/src/foundation/diagnostics.dart

@@ -3024,8 +3024,12 @@ class DiagnosticPropertiesBuilder {
 /// string and [DiagnosticsNode] debug representations describing the properties
 /// of an object.
 ///
-/// _This mixin is exists only to support plugins that require older Flutter
-/// versions: Use the identical mixin [Diagnosticable] instead for most code._
+/// _This mixin exists only to support plugins and packages that require older
+/// Flutter versions: Use the identical mixin [Diagnosticable] instead for all
+/// new code. If you are authoring code that needs to work on the stable branch
+/// as well as master (in a package, for instance), mix this in instead of
+/// extending [Diagnosticable]. Once [Diagnosticable] as a mixin reaches the
+/// stable channel, this class will be deprecated._
 mixin DiagnosticableMixin implements Diagnosticable {
   @override
   String toStringShort() => describeIdentity(this);

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

@@ -2054,7 +2054,7 @@ abstract class BuildContext {
 
   /// Whether the [widget] is currently updating the widget or render tree.
   ///
-  /// For [StatefullWidget]s and [StatelessWidget]s this flag is true while
+  /// For [StatefulWidget]s and [StatelessWidget]s this flag is true while
   /// their respective build methods are executing.
   /// [RenderObjectWidget]s set this to true while creating or configuring their
   /// associated [RenderObject]s.