docs: update services description for concept guide
docs: update services description for concept guide

This patch updates the API concept guide about services.

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

Change-Id: I9196cb6b700687d2a42d4bf32ecb02a7300a9475

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