Revise description of built-in namespaces
Revise description of built-in namespaces

jonfoster/pyxb2,jonfoster/pyxb2,CantemoInternal/pyxb,jonfoster/pyxb-upstream-mirror,jonfoster/pyxb1,jonfoster/pyxb1,jonfoster/pyxb2,CantemoInternal/pyxb,balanced/PyXB,pabigot/pyxb,balanced/PyXB,CantemoInternal/pyxb,jonfoster/pyxb-upstream-mirror,jonfoster/pyxb-upstream-mirror,balanced/PyXB,pabigot/pyxb