Added detailed comment to document the nested-ref hack in a34dfb15

Change-Id: Ia4fafb3394e3489743a99ca25b7a1dfc5c50aaa4
(Bug 49729) Update DSR for all nested DOM nodes of <figure>

* It is not sufficient to update DSR on the top-level figure node.
  The DSR values for nested elements in the figure (ex: caption)
  will be incorrect by the delta between the old and new tsr.
  So, the DSR values have to be updated by that amount for all
  nested elements.

Change-Id: I32c511d05d62dd31bb19fca21aff792ddc422331