ast/module: Additional documentation for type methods

Bug: tint:337
Change-Id: I60dae81baba3c5c85efa277e772f4a1f12589e11
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/33786
Auto-Submit: Ben Clayton <bclayton@google.com>
Commit-Queue: David Neto <dneto@google.com>
Reviewed-by: David Neto <dneto@google.com>

src/ast/module.h

@@ -101,8 +101,13 @@ class Module {
   /// Creates a new `ast::Type` owned by the Module.
   /// When the Module is destructed, owned Module and the returned
   /// `ast::Type` will also be destructed.
-  /// Types are unique (de-aliased), and so `create()` for the same `T` and
+  /// Types are unique (de-aliased), and so calling create() for the same `T`
-  /// arguments will return the same pointer.
+  /// and arguments will return the same pointer.
+  /// @warning Use this method to acquire a type only if all of its type
+  /// information is provided in the constructor arguments `args`.<br>
+  /// If the type requires additional configuration after construction that
+  /// affect its fundamental type, build the type with `std::make_unique`, make
+  /// any necessary alterations and then call unique_type() instead.
   /// @param args the arguments to pass to the type constructor
   /// @returns the de-aliased type pointer
   template <typename T, typename... ARGS>
@@ -116,6 +121,7 @@ class Module {
   /// (de-aliased) type.
   /// When the Module is destructed, the returned `ast::Type` will also be
   /// destructed.
+  /// @see create()
   /// @param ty the type to add to the module
   /// @returns the de-aliased type pointer
   template <typename T>