LiveHTML code comments/cleanups

@dangoor - see notes on my second commit as well.

I encountered another case (in `_handleDeletions()`) where we were scanning the whole document for marks, so I'm still feeling a little icky about leaving that in. But I added in an optimization to avoid doing so if there are no deletions, which is the common case for typing. Probably the right thing to do is to combine `_handleDeletions()` with `_updateMarkedRanges()`, but I'm just leery enough of fiddling with the logic at this point without a clear need to do so.