Small change to 'is_public' documentation
Small change to 'is_public' documentation

Update the documentation for 'is_public' in authentication.rst
to be more accurate.

Addresses bug 1177732.

Change-Id: I9889826d4019ff33d9be343e79324862d9fde8e9

redhat-openstack/glance,akash1808/glance,vuntz/glance,ntt-sic/glance,klmitch/glance,citrix-openstack-build/glance,wkoathp/glance,jumpstarter-io/glance,ozamiatin/glance,takeshineshiro/glance,stevelle/glance,openstack/glance,saeki-masaki/glance,dims/glance,redhat-openstack/glance,tanglei528/glance,stevelle/glance,sigmavirus24/glance,akash1808/glance,paramite/glance,cloudbau/glance,ozamiatin/glance,saeki-masaki/glance,citrix-openstack-build/glance,scripnichenko/glance,klmitch/glance,tanglei528/glance,SUSE-Cloud/glance,SUSE-Cloud/glance,scripnichenko/glance,vuntz/glance,rajalokan/glance,rickerc/glance_audit,paramite/glance,JioCloud/glance,sigmavirus24/glance,darren-wang/gl,darren-wang/gl,jumpstarter-io/glance,JioCloud/glance,openstack/glance,kfwang/Glance-OVA-OVF,ntt-sic/glance,kfwang/Glance-OVA-OVF,wkoathp/glance,cloudbau/glance,rajalokan/glance,openstack/glance,takeshineshiro/glance,dims/glance,rickerc/glance_audit