Reconsider guest access on new room presets

> Isn't it a little counter-intuitive that in a private chat guests are allowed but not in the public preset?
> I would suggest to default to forbidden for all the presets.

([source](https://github.com/matrix-org/matrix-doc/pull/1379#issuecomment-404733027))

Start with the linked Matrix spec pull request and its referenced discussion about guest access in private and public room presets. Determine the intended default for each preset and document the agreed behavior in the relevant room-preset specification; done means the defaults are consistent and the specification no longer leaves the behavior ambiguous.

Written by the indexing model from the issue text.