Only show test cookie warnings on submit as caching/proxies may intercept the test cookie for GET requests.

Introduce a new string for when headers are sent and link them to http://codex.wordpress.org/Cookies (new page).

props SergeyBiryukov.
fixes #27373.


git-svn-id: https://develop.svn.wordpress.org/trunk@27859 602fd350-edb4-49c9-b593-d223f7449a82