DOCS-3711

Clarify different procedure for adding first user admin or root user
I'm wary of beginning sentences with "note."  Seems unnecessary, and doesn't actually make things easier. Our general rule is, if it's a note, use a note box, and if it's in a note box, make sure that it is truly exceptional (in most cases its not, and we should probably use page structure to more clearly draw attention to points like this.)

I'm also worried about the generic and somewhat ambiguous use of the term database here. 

I'd probably rewrite this as: 

> If you have defined any users for the `mongod` instance, but no user has the permission to create a user, you *must* restart the `mongod` instance with the `--noauth` option to create new users.