doc: use "303 See Other" redirects after POST

Per RFC 2616 (HTTP 1.1), the HTTP response status code "303 See Other"
is the correct way to redirect a client after an HTTP POST. Adjust the
documentation to use a 303 redirect instead of 302.
Fixing returns in PHPdoc

My IntelliJ (IDE) complains in the method handler() because the PHPdoc for Kohana_Exception::_handler() told it that a boolean was returned. The "boolean" was then used as an Response object. Since a Response was the actual returned value, I fixed the PHPdoc.
Merge pull request #373 from HNygard/patch-1

Fixing returns in PHPdoc

Refs #4762
Merge pull request #343 from ktdreyer/3.3/validation-303-redirect

doc: use "303 See Other" redirects after POST

Refs #4720