added info to section_intro-namespaces
added info to section_intro-namespaces

documentation for namespaces

Change-Id: I93bd9d21e8a77f6fbef3b5937f7fec1320a884b8

kairoaraujo/openstack-manuals,Akasurde/openstack-manuals,saeki-masaki/openstack-manuals,kairoaraujo/openstack-manuals,openstack/openstack-manuals,gongysh2004/openstack-manual,saeki-masaki/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,Akasurde/openstack-manuals,openstack/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,potsmaster/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,gongysh2004/openstack-manual,openstack/openstack-manuals,potsmaster/openstack-manuals,openstack/openstack-manuals