Rename default Admin role to Cluster Admin

Clarify the purpose and distinquesh the Admin role a bit better from the
in-game admin status by renaming the role to Cluster Admin.

docs/how-it-works.md

@@ -27,4 +27,4 @@ that log into instances under its control.  These accounts are used to
 store per player data and authorize management of the cluster.  By
 default users accounts are assigned a Player role that gives a limited
 read access to the cluster resources, and admins of the cluster can be
-assigned an Admin role to get access to manage it.
+assigned a Cluster Admin role to get access to manage it.

docs/managing-a-cluster.md

@@ -107,7 +107,7 @@ Replaces the roles the user has in the cluster with a new list of roles.
 Calling with an empty roles argument will remove all roles from the
 user.
 
-By default there's a roled named Admin which grants access to
+By default there's a roled named Cluster Admin which grants access to
 everything and a role named Player which grants a limited read access to
 the cluster, see [the section on roles](#roles) for more information
 about setting up roles and permissions.