[api] Add documentation for v8::Platform::GetPageAllocator

Change-Id: I5760687fa9e8a7512d3946704f51f321ffa3676b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3963718
Commit-Queue: Adam Klein <adamk@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#83827}

include/v8-platform.h

@@ -923,6 +923,7 @@ class Platform {
 
   /**
    * Allows the embedder to manage memory page allocations.
+   * Returning nullptr will cause V8 to use the default page allocator.
    */
   virtual PageAllocator* GetPageAllocator() = 0;