Disable UnusedMethodArgument for abstract methods

Conflict!

* [Rubocop](https://github.com/bbatsov/rubocop) tells us to prefix unused variables with an underscore. This makes sense, and recently, in PR #500, I made some style changes to the nanoc source code to please Rubocop.

* [YARD](http://yardoc.org/) uses parameter names in its `@param` annotations. When doing the rename mentioned above, these parameter names got out of sync with the actual source code. From a documentation point of view, having underscore parameter names is ugly.

The solution: tag abstract methods with `# rubocop:disable Lint/UnusedMethodArgument` to pacify Rubocop.

(I also fixed a `@param` that should have been a `@return`, so that now both Rubocop and YARD are happy.)