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.

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git-svn-id: http://core.svn.wordpress.org/trunk@27690 1a063a9b-81f0-0310-95a4-ce76da25c4cd