Add documentation comments to public declarations

[mattt]

Surge functionality is, for the most part, "self-documenting". Functions follow standard naming conventions and take typed LHS / RHS arguments in such a way that anyone familiar with the domain shouldn't have much trouble understanding how anything works.

However, providing documentation comments would add support for Xcode help and allow tools like swift-doc to generate a formal API reference. The current approach of organizing functions under MARK headers provides some benefit, but having proper documentation comments would be an improvement.