Make it clear which built-in roles are required to run each command

Many users have reported that it's not that obvious/memorable for example that this:

http://docs.mongodb.org/manual/reference/user-privileges/#clusterAdmin
...links to this....
http://docs.mongodb.org/manual/reference/command/listDatabases/#dbcmd.listDatabases
...which means you need clusterAdmin privileges to run "show dbs" in the mongo shell

I think this translates into the following improvements needed to the documentation:

• Should clarify somewhere the Access Control documentation which roles are required to run various shell helper commands such as:
  • show users
  • show dbs
  • show collections
• Add a note to the documentation page for each command that links back to the built-in role(s) that confer the privilege to run that command.