[DX] Support #attributes in theme_help() and theme_more_help_link()

The '#type' => 'help' element is a nice utility for displaying help text wrapped in <p>...</p> tags and it's widely used in core. It would be nice if it supported the #attributes property, so that, for example, one could easily add classes to the <div> wrapper. (And then, for consistency, theme_more_help_link() should also support #attributes.) It would be relatively easy to add, and since it's never been supported before, seems unlikely to break anything by its addition. What do people think?