Comments can be added to the XML but not at the moment in the full editor.
For example I added this:
<Transition>2</Transition>
</Entry>
<!-- Comment #1 -->
<Entry>
<Index>0x1A00</Index>
<SubIndex>0</SubIndex>
<Length>1</Length>
<Data>0</Data>
<Flags>NONE</Flags>
<Transition>2</Transition>
</Entry>
<Entry>
<Index>0x1600</Index>
And it appears in the comment field of the editor.