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.
Merge pull request #343 from ktdreyer/3.3/validation-303-redirect

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