updated to include cluster list docs

@thousandsofthem here go
just thinking `specially assigned` should be available to anyone, even "normal" users. 
Only admin can assign these special clusters -> no potential issues
Also, can imagine some usecases:
* Customer X ("normal" free or low-level plan) want to do battletesting before upgrading
* Quick test for a fix. add cluster to a user, test it, roll out for everyone
* Private beta program.