More flexible documentation for R6 methods

[hadley]

• Parse calls like object$set() so you can document dynamic method generation. Fixes Documentation for R6 methods added dynamically by "$set()" #931
• New @R6method tag as a generic way of documenting any R6 method. Fixes Document R6 methods created on $new()/$initialize() #991.

[gaborcsardi]

Makes sense. AFAICT this does not solve one of the original issues (#931) because there the methods are added with $set() dynamically in a loop. OTOH that issue is probably solved best with dynamically generating the documentation as well, using `r ...`.

I am a bit worried about the edge cases, e.g. what if people put @R6method inside the R6 class, right before the method?

[hadley]

I think if you're dynamically generating the methods, you could still dynamically generate the roxygen comments, either by writing to a file or (maybe) with @eval (depending on exactly when that's run in the process).

I probably won't merge this PR because I've decided it makes sense to tackle a bigger refactoring first: I want to add an intermediate layer of rd_ objects to make it easier to test generating the R6 object model vs generating the .Rd output.

[hadley]

Superseded by #1824