Fix doxygen warnings about missing docs

Change-Id: I6bfade1db4b3e1a8c63a0bba16303925554ca6cc
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/34569
Reviewed-by: dan sinclair <dsinclair@chromium.org>
Commit-Queue: Ben Clayton <bclayton@google.com>

src/ast/module.h

@@ -37,12 +37,18 @@ class Module {
       typename std::enable_if<std::is_base_of<BASE, T>::value, T>::type;
 
  public:
+  /// Constructor
   Module();
+
   /// Move constructor
   Module(Module&&);
-  /// Move assignment
+
+  /// Move assignment operator
   /// @param rhs the Module to move
+  /// @return this Module
   Module& operator=(Module&& rhs);
+
+  /// Destructor
   ~Module();
 
   /// @return a deep copy of this module

src/ast/type_manager.h

@@ -28,12 +28,18 @@ namespace ast {
 /// The type manager holds all the pointers to the known types.
 class TypeManager {
  public:
+  /// Constructor
   TypeManager();
+
   /// Move constructor
   TypeManager(TypeManager&&);
-  // Move assignment
+
+  /// Move assignment operator
   /// @param rhs the TypeManager to move
+  /// @return this TypeManager
   TypeManager& operator=(TypeManager&& rhs);
+
+  /// Destructor
   ~TypeManager();
 
   /// Clears all registered types.