[Async DB] Documentation/minor tweaks

I'm adding documentation, making minor tweaks as I'm reading through the code. Some of the changes are likely unnecessary/incorrect as I may be overlooking some finer aspects of the new code.

Let me know if I should continue with this or not.

I just started, so I plan to push more as I go through the code (planning to get through all the data code this weekend). I try to make sure nothing breaks, but I also haven't tested this code yet.

(I see you addressed the comments I made, so I'll have to rebase this anyway)
I was just thinking (I imagine for very similar reasons as you) that a stack flag to ```Zotero.debug()``` would be helpful — but unless I'm misunderstanding this, this seems to require passing in an actual stack and level? I think we should just take a boolean to add a stack trace to an arbitrary ```Zotero.debug()``` call.