Update documentation on HTTP Exceptions

Issue https://github.com/Pylons/webob/issues/169 explains why this change is nice, there is no requirement that these are WebDAV only, and in fact many REST services use `422` to signal to the client that they sent properly formatted data, but that it was not processable as it stands and needs to be modified (possibly missing attributes or information in the JSON/XML for example).

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Also updated the documentation at the top of the document, it did not include some of the errors, that has now been rectified.