Fixes #3016

Clarify in the doc that tracking_requests_require_authentication should not be used on public facing Piwik servers since it would allow anyone to push data in the past, future, or with custom IP, which is a security concern

git-svn-id: http://dev.piwik.org/svn/trunk@5978 59fd770c-687e-43c8-a1e3-f5a4ff64c105