Document the Access Roles Graylog need in MongoDB

[HenryTheSir]

Context

When you run Graylog with secured MongoDB and user authentication you need to give the Graylog user the proper rights.

This needs to be added to https://docs.graylog.org/en/4.0/pages/installation.html#system-requirements

Expected Content

Detailed information about which permissions the user in Mongodb needs.

[jalogisch]

This is already mentioned but only over here: https://docs.graylog.org/en/4.0/pages/configuration/multinode_setup.html#mongodb-replica-set