Added clarficiation for occurences and refresh

This is a FAQ

https://groups.google.com/forum/#!searchin/sensu-users/mail$20occurences/sensu-users/nQKrhqvdWe0/7W0DI1y2oecJ
https://groups.google.com/forum/#!searchin/sensu-users/mail$20occurences/sensu-users/X8WgIuWSs20/BY39X8zU0gIJ
https://groups.google.com/forum/#!topic/sensu-users/bBXs8ZDjBZw