Update documentation for exceptions

This was brought up by https://github.com/Pylons/webob/issues/169 but is something I was thinking earlier this week as well while building my latest app.

There is no reason why these HTTP Exceptions should be labeled WebDAV only, especially since browsers treat them correctly even if it is not WebDAV. This also makes sense for REST API's where having at least 422 (Unprocessable Entity) be available makes a lot of sense because it gives a better signal back to the client than 400 (Bad Request).