Provide more elaborate explanation of the roles in the documentation

[zveljanoski]

The tables in the documentation (https://docs.gravitee.io/am/current/am_adminguide_roles_and_permissions.html) basically not providing any information about what these roles and their permissions actually control, most of them just say "Manages {name of the role}".

Use case: creating roles for the users of the platform. This requires to understand how to turn a button and its functionality on/off. It is currently very hard to grasp how to set these permissions, which permissions are necessary for which actions etc.