Added note to help differentiate MINIO creds from S3 rotating creds (#9887)

docs/gateway/s3.md

@@ -22,6 +22,8 @@ minio gateway s3
 
 ### Using Binary in EC2
 Using IAM rotating credentials for AWS S3
+
+If you are using an S3 enabled IAM role on an EC2 instance for S3 access, MinIO will still require env vars MINIO_ACCESS_KEY and MINIO_SECRET_KEY to be set for its internal use. These may be set to any value which meets the length requirements. Access key length should be at least 3, and secret key length at least 8 characters.
 ```
 export MINIO_ACCESS_KEY=custom_access_key
 export MINIO_SECRET_KEY=custom_secret_key