Feature request: Reuse documentation comments for protocols on implementation.

[galli-leo]

Not sure if this is doable, but when declaring a protocol that is used by many classes, it gets annoying having to copy paste most of the documentation. Thus I would propose an option, which uses the documentation of a function / property from the protocol itself, if no doc comment is on the actual implementation.

Example:

protocol MyProtocol {
  /// Does awesome stuff!
  func myAwesomeFunc()
}

extension String: MyProtocol {
  func myAwesomeFunc() {
    self = "awesome"
  }
}

Jazzy could then create the following documentation info on myAwesomeFunc() for String: "Does awesome stuff!"

[johnfairh]

This is oft-requested, tracked by #190. Implementation not entirely straight-forward which is why it hasn't been done yet!