Make /document API variable names (and pointer-ness) more consistent

[MaddieM4]

Right now, we have a lot of variation in the argument names for the document.Event functions. Less so for document.Quorum, but there's still a weird mix of pointer receivers and copy receivers.

I'd like to make that all consistent, so it lines up nicely, and pointer-ness is never surprising or unpredictable. This isn't urgent, but it's also not hard to do, either (no real functionality changes), so the only excuse not to do it is higher-priority tickets stealing my attention away.

This is also hypothetically something that an external developer could do right now! Which is incredibly exciting, that the code is finally sane enough that I feel like other people can work on it in parallel. I'll need to add a new label to the issue tracker 😄