Show on the segmentation doc that a segment is only accessible by an admin user (this is only the case for visitIp segment for now)
visitorId can now be used by anyone, because visitorId is only displayed in the Live! API to authenticated users anyway

git-svn-id: http://dev.piwik.org/svn/trunk@4465 59fd770c-687e-43c8-a1e3-f5a4ff64c105