DOCS-3711

Clarify different procedure for adding first user admin or root user
I'm wary of relative references (e.g. step 3) to other parts of content, particularly in includes files. This file is used six different files. 

Also I'm not sure that this needs to be inside of a note block. It's not more special than the other items discussed in the file. 

You can check for include-file use with the following operation: 

    $ giza includes graph -f /includes/access-create-user.rst 
    {
        "/includes/access-create-user.rst": [
           "/reference/command/createUser.txt", 
           "/reference/method/db.createUser.txt", 
           "/tutorial/add-admin-user.txt", 
           "/tutorial/add-user-administrator.txt", 
           "/tutorial/add-user-to-database.txt"
         ]
     }