ActivityPubController: Update upload_media @doc

Small cherry-pick from https://git.pleroma.social/pleroma/pleroma/-/merge_requests/1810

lib/pleroma/web/activity_pub/activity_pub_controller.ex

@@ -514,7 +514,6 @@ defp ensure_user_keys_present_and_maybe_refresh_for_user(user, for_user) do
     {new_user, for_user}
   end
 
-  # TODO: Add support for "object" field
   @doc """
   Endpoint based on <https://www.w3.org/wiki/SocialCG/ActivityPub/MediaUpload>
 
@@ -525,6 +524,8 @@ defp ensure_user_keys_present_and_maybe_refresh_for_user(user, for_user) do
   Response:
   - HTTP Code: 201 Created
   - HTTP Body: ActivityPub object to be inserted into another's `attachment` field
+
+  Note: Will not point to a URL with a `Location` header because no standalone Activity has been created.
   """
   def upload_media(%{assigns: %{user: %User{} = user}} = conn, %{"file" => file} = data) do
     with {:ok, object} <-