The Atlas FAQ and documentation does not appear to describe what happens when the storage limit for the current tier is reached:

• For M0 (free tier) the maximum storage size is a hard limit. Additional storage can be added by upgrading to a paid cluster.

• Paid clusters will auto-expand storage by default based on disk usage thresholds. This setting can be disabled if a fixed storage limit is preferable.

I suggest including expected error message and behaviour as well as a link to the DB Storage alerts that can be set.

Example error message:

WriteResult({
  "writeError": {
    "code": 8000,
    "errmsg": "you are over your space quota, using 513 MB of 512 MB"
  }
})