DOC: Update information on start_date & end_date
DOC: Update information on start_date & end_date

Add information on start_date and end_date.  Explain the values that the
parameters take.

Change-Id: Id41a88c4e10f9e020c25a893b6986119e8711f76

Solinea/stackalytics,mhharsh/stackalytics,Code-Fly/stackalytics,AlexanderYAPPO/stackalytics,openstack/stackalytics,mhharsh/stackalytics,Mirantis/stackalytics,mhharsh/stackalytics,stackforge/stackalytics,openstack/stackalytics,Mirantis/stackalytics,Code-Fly/stackalytics,danielmellado/stackalytics,dliu2015/stackalytics_zanata,danielmellado/stackalytics,stackforge/stackalytics,0xf2/stackalytics,dliu2015/stackalytics_zanata,dliu2015/stackalytics_zanata,openstack/stackalytics,stackforge/stackalytics,0xf2/stackalytics,mohitsethi/stackalytics,danielmellado/stackalytics,Code-Fly/stackalytics,AlexanderYAPPO/stackalytics,0xf2/stackalytics,Solinea/stackalytics,Mirantis/stackalytics,mohitsethi/stackalytics,Solinea/stackalytics,AlexanderYAPPO/stackalytics,0xf2/stackalytics,openstack/stackalytics,mohitsethi/stackalytics