Add missing documentation to Viewport

[Mickeon]

This PR fills all of the class reference's descriptions that have been missing from Viewport. It also tweaks existing descriptions very slightly.

In particular, the descriptions for sdf_oversize, sdf_scale, snap_2d_transforms_to_pixel, and snap_2d_vertices_to_pixel have almost been copied word-for-word from the equivalent Project Settings.

I could have rewritten them outright, but the time I spend on these is directly proportional to how long it will take for them to be reviewed and merged.

[mhilbrunner]

@Mickeon This looks good to me, but needs a rebase.

[akien-mga]

Thanks!