BRegion: Style updates for documentation.

No functional changes intended.

* Updated copyright information.
* Reduced doxygen documentation down to a helpful summary
  in a regular comment, the documentation has been moved into
  the Haiku Book.
* Some parameter renaming for consistency and clarity.
* A few other style fixes.
Add BRegion documentation.

Most was written by Stephan Aßmus, and Stefano Ceccherini.
(moved from Region.cpp)

Thank you both for your contributions.