Clearer definition of PaaS
Clearer definition of PaaS

Replace a potentially misleading example and add a bit of detail to the
definition.

Change-Id: I9b80b693d8c36742e20f04e8facbdff3dbf0838e

openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals