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.