Fixing default UAA admin authorities

As to avoid access_denied when trying to create a new user.
See related thread at https://groups.google.com/a/cloudfoundry.org/d/msg/vcap-dev/q9HlwPAPBTw/Aa3slgliuu8J
Adding pointers to UAA docs while managing uers

both UAA admin guide and UAA architecture into in cf-docs
Merge pull request #293 from Orange-OpenSource/uaa-fix-admin-perm

Fixing default UAA admin authorities