[api] Slightly improve ValueSerializer documentation

Without specifying what the default allocator does, using
the buffer returned from `Release()` means that one basically
had to make an educated guess on how to free it
(and that ownership actually was transferred to the caller).

Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: Ie2ac0c91d9bdafbe91f6bce9b2263e304b2336ea
Reviewed-on: https://chromium-review.googlesource.com/1056369Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53225}

include/v8.h

@@ -2025,12 +2025,16 @@ class V8_EXPORT ValueSerializer {
      * If the memory cannot be allocated, nullptr should be returned.
      * |actual_size| will be ignored. It is assumed that |old_buffer| is still
      * valid in this case and has not been modified.
+     *
+     * The default implementation uses the stdlib's `realloc()` function.
      */
     virtual void* ReallocateBufferMemory(void* old_buffer, size_t size,
                                          size_t* actual_size);
 
     /**
      * Frees a buffer allocated with |ReallocateBufferMemory|.
+     *
+     * The default implementation uses the stdlib's `free()` function.
      */
     virtual void FreeBufferMemory(void* buffer);
   };
@@ -2058,9 +2062,9 @@ class V8_EXPORT ValueSerializer {
 
   /**
    * Returns the stored data (allocated using the delegate's
-   * AllocateBufferMemory) and its size. This serializer should not be used once
-   * the buffer is released. The contents are undefined if a previous write has
-   * failed.
+   * ReallocateBufferMemory) and its size. This serializer should not be used
+   * once the buffer is released. The contents are undefined if a previous write
+   * has failed. Ownership of the buffer is transferred to the caller.
    */
   V8_WARN_UNUSED_RESULT std::pair<uint8_t*, size_t> Release();