Better description of available RBAC roles in EE vs CE

During debugging of the a forum user's problem creating an RBAC user, it was found that the list of roles differs between EE and CE but the documentation does not clearly show what is available in each edition.

Forum post: https://forums.couchbase.com/t/trouble-creating-a-user/16772
Docs page: https://developer.couchbase.com/documentation/server/5.1/security/concepts-rba-for-apps.html

NOTE: I created the following PR to correct the full bucket access role name on the same page