Expand Semantics dartdocs. (#6399)

I based this on a conversation I had with @mpcomplete.

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

@@ -2694,10 +2694,26 @@ class MetaData extends SingleChildRenderObjectWidget {
 
 // UTILITY NODES
 
-/// A widget that annotates the widget tree with a description of the meaning of the widgets.
+/// A widget that annotates the widget tree with a description of the meaning of
+/// the widgets.
 ///
 /// Used by accessibility tools, search engines, and other semantic analysis
 /// software to determine the meaning of the application.
+///
+/// See also:
+///
+/// * [MergeSemantics], which marks a subtree as being a single node for
+///   accessibility purposes.
+/// * [ExcludeSemantics], which excludes a subtree from the semantics tree
+///   (which might be useful if it is, e.g., totally decorative and not
+///   important to the user).
+/// * [RenderObject.semanticsAnnotator], the rendering library API through which
+///   the [Semantics] widget is actually implemented.
+/// * [SemanticsNode], the object used by the rendering library to represent
+///   semantics in the semantics tree.
+/// * [SemanticsDebugger], an overlay to help visualize the semantics tree. Can
+///   be enabled using [WidgetsApp.showSemanticsDebugger] or
+///   [MaterialApp.showSemanticsDebugger].
 class Semantics extends SingleChildRenderObjectWidget {
   /// Creates a semantic annotation.
   ///