[#5517] add permission_desc docstring; remove caching
[#5517] add permission_desc docstring; remove caching

Caching can be incorrect if it's called on a class in the middle of the hierarchy.
Any subclasses after it won't have their descriptions merged in

apache/incubator-allura,lym/allura-git,lym/allura-git,heiths/allura,lym/allura-git,apache/incubator-allura,apache/allura,heiths/allura,heiths/allura,apache/allura,lym/allura-git,apache/allura,heiths/allura,apache/incubator-allura,apache/allura,lym/allura-git,heiths/allura,apache/allura,apache/incubator-allura