api-guide: add note about users
api-guide: add note about users

In the introduction to the guide, note that there is a single API for
both end users and operators.

blueprint complete-todo-in-api-concept-doc

Change-Id: I726d1f230c0090ca8890195f56167e19a18e4665

jianghuaw/nova,cernops/nova,rajalokan/nova,rahulunair/nova,phenoxim/nova,openstack/nova,rajalokan/nova,NeCTAR-RC/nova,zhimin711/nova,rahulunair/nova,Juniper/nova,BeyondTheClouds/nova,alaski/nova,openstack/nova,cloudbase/nova,vmturbo/nova,jianghuaw/nova,klmitch/nova,cernops/nova,bigswitch/nova,cloudbase/nova,klmitch/nova,mikalstill/nova,cyx1231st/nova,cyx1231st/nova,vmturbo/nova,sebrandon1/nova,vmturbo/nova,klmitch/nova,Juniper/nova,rajalokan/nova,vmturbo/nova,openstack/nova,Juniper/nova,zhimin711/nova,dims/nova,sebrandon1/nova,mikalstill/nova,cloudbase/nova,rajalokan/nova,jianghuaw/nova,phenoxim/nova,alaski/nova,apporc/nova,NeCTAR-RC/nova,gooddata/openstack-nova,klmitch/nova,Juniper/nova,jianghuaw/nova,cernops/nova,dims/nova,bigswitch/nova,BeyondTheClouds/nova,hanlind/nova,gooddata/openstack-nova,apporc/nova,mikalstill/nova,sebrandon1/nova,BeyondTheClouds/nova,gooddata/openstack-nova,hanlind/nova,CEG-FYP-OpenStack/scheduler,rahulunair/nova,hanlind/nova,mahak/nova,gooddata/openstack-nova,mahak/nova,mahak/nova,CEG-FYP-OpenStack/scheduler