cppgc: Add example for AdditionalBytes

Bug: chromium:1056170
Change-Id: I19c7d69c53cb7f4a2b8400ec31c63e88496c8982
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2532304
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Auto-Submit: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71131}

include/cppgc/allocation.h

@@ -112,11 +112,29 @@ class MakeGarbageCollectedTraitBase
 };
 
 /**
- * struct used specify to MakeGarbageCollected how many bytes should be
- * appended to the allocated object.
+ * Passed to MakeGarbageCollected to specify how many bytes should be appended
+ * to the allocated object.
+ *
+ * Example:
+ * \code
+ * class InlinedArray final : public GarbageCollected<InlinedArray> {
+ *  public:
+ *   explicit InlinedArray(size_t bytes) : size(bytes), byte_array(this + 1) {}
+ *   void Trace(Visitor*) const {}
+
+ *   size_t size;
+ *   char* byte_array;
+ * };
+ *
+ * auto* inlined_array = MakeGarbageCollected<InlinedArray(
+ *    GetAllocationHandle(), AdditionalBytes(4), 4);
+ * for (size_t i = 0; i < 4; i++) {
+ *   Process(inlined_array->byte_array[i]);
+ * }
+ * \endcode
  */
 struct AdditionalBytes {
-  explicit AdditionalBytes(size_t bytes) : value(bytes) {}
+  constexpr explicit AdditionalBytes(size_t bytes) : value(bytes) {}
   const size_t value;
 };