Mark preview_image_needed_before_processing_variants? as private API

It looks like `preview_image_needed_before_processing_variants?` was added recently,
but it's stated as being public API.

However, it looks like it's more of an implementation detail that's not meant for Active Storage users.

So this marks it as nodoc, so we're not on the hook for maintaining it.

activestorage/app/models/active_storage/blob/representable.rb

@@ -98,7 +98,7 @@ def representable?
     variable? || previewable?
   end
 
-  def preview_image_needed_before_processing_variants?
+  def preview_image_needed_before_processing_variants? # :nodoc:
     previewable? && !preview_image.attached?
   end