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.

This is filed in Redmine as  http://dev.kohanaframework.org/issues/4720

Previously erroneously submitted against 3.3/master as #339 . In that ticket, @zombor pointed out that there may have been a historic reason for choosing 302 explicitly. From Googling around, it appears that pre-HTTP/1.1 clients may have not supported 303 redirects in the past. However, it is a valid HTTP/1.1 operation, and speaking anecdotally, I've been doing this in my own web applications with no problem. I've been testing on Firefox, Chromium, and mobile Safari.