Document dialog options

@marijnh Here's some docs for `openDialog` and `openNotification`. As I wrote the docs for the latter, I realized `openNotification` doesn't actually set the 5000ms default timeout if you pass no options argument at all. So I fixed that too.

Wasn't sure if you wanted `openConfirm` documented or if it's meant to be an internal thing.

Anyway, let me know if there's anything I can do to improve the formatting.