[MEKO] Clarify that you can assign a user to only one MongoDB resource

Description

Per https://docs.mongodb.com/kubernetes-operator/stable/tutorial/manage-database-users-scram/#k8s-manage-db-users-scram, it implies that only one user (awong) can belong in a single MongoDB resource (my-replica-set4).    

apiVersion: mongodb.com/v1
kind: MongoDBUser
metadata:
name: mms-scram-awong
spec:
passwordSecretKeyRef:
name: awong-password

1. Match to metadata.name of the User Secret
   key: password
   username: "awong"
   db: "admin" #
   mongodbResourceRef:
   name: "my-replica-set4"
2. Match to MongoDB resource using authenticaiton
   roles:

• db: admin
  name: clusterAdmin
• db: admin
  name: dbAdminAnyDatabase
• db: admin
  name: userAdminAnyDatabase
• db: admin
  name: readWriteAnyDatabase

I tried to assign this user to my-replica-set4 and my-replica-set5 and it wouldn't let me.   Is it possible? If so, can we update the instructions?

Scope of changes

Impact to Other Docs

MVP (Work and Date)

Resources (Scope or Design Docs, Invision, etc.)