Notifications support several options that were yet to be documented:
`buttons`, `description`, and `stack`. Add descriptions for each option
and its defaults (where applicable, e.g. .buttons.className).
`stack` is technically available on all notification types, but it
doesn't make sense to use it on types other than 'error' and 'fatal'.
Therefore it's documented only in those two cases.
The usage and expected display, like preformatted vs. Markdown, taken
from [atom/notifications/lib/notification-element.coffee][1].
[1]: https://github.com/atom/notifications/blob/v0.64.1/lib/notification-element.coffee
