Fix TODO comments in class reference

[RedMser]

Fixes #65576
(I suggest closing the tracker issue, so that future TODOs don't end up as remarks in the text, but instead as issues on the repository)

• RenderingServer.viewport_attach_to_screen docs were confused about why the method does not exist on Viewport. I just rewrote the code sample to use the API itself, and verified that it still does exactly what the description above is describing.
• SurfaceTool.commit docs did not understand the flags parameter. Since the custom format flags seem to be filled automatically, only the compress flags can be specified. I took the possible values from ArrayMesh.add_surface_from_arrays, even though this only lists some of the compress flags and not a full exhaustive list. I assume there's good reason for this, but maybe not?
• While touching RenderingServer docs, I took the liberty of documenting duplicate enum values that are already documented in the Mesh class.

[Mickeon]

Approving on the content of it, but the rendering team needs to take a look.
Note that, indeed, the descriptions of these constants are duplicates of existing ones. Any suggestions for them should also apply to the originals, hence I'm not proposing changes to them here.

[akien-mga]

Thanks!