Put arity functions into separate scaladoc groups

[ceedubs]

Currently certain scaladocs (such as Apply) contain a whole bunch of generated methods of various arities. This makes it really tough to find the "core" methods that people are probably interested in. I think that we should move the arity methods (at least for 3 or 4 and above) into a separate scaladoc @groups.

One downside is that methods in groups cannot be linked to. However I think not being able to link to these methods is not going to be a problem, and this probably isn't a hard scala bug to fix, so one of us should just fix it :)

cc @mpilquist who may be the only scala dev I know who is good about using scaladoc groups

[ceedubs]

I have this working on a local branch that I need to get around to turning into a PR.

[ceedubs]

Resolved by #2080.