add docs to CollectionProxy

[frodsan]

Extended from https://github.com/lifo/docrails/commit/3c91c8127050c7abbe091c19052f9813a62b1af7#commitcomment-1346945

[frodsan]

cc @jonleighton @vijaydev @fxn If this ok, i will remove the changes in docrails and are more docs here :).

[jonleighton]

Looks good to me, but I will wait for @fxn and @vijaydev to weigh in if they have thoughts on this approach.

[drogus]

@frodsan there is also another method of adding docs to methods defined at runtime: 7dba512

[frodsan]

@drogus nice, thanks. I will make the changes in docrails.

[frodsan]

I made this change in docrails : https://github.com/lifo/docrails/commit/1b5c775e7a6d5457da6d2fe02e9e72c6788a7373

[jonleighton]

TBH I prefer the explicit delegation version. It makes the arity of the delegation method explicit.

[frodsan]

TBH i agree. It's up to you guys.