rdoc 2.1.0

RDoc is an application that produces documentation for one or more Ruby source files. RDoc includes the `rdoc` and `ri` tools for generating and displaying online documentation. At this point in time, RDoc 2.x is a work in progress and may incur further API changes beyond what has been made to the RDoc 1.0.1. Command-line tools are largely unaffected, but internal APIs may shift rapidly.