add student-facing note that where there are multiple submissions, only ...

...the highest is counted (ORA-209)

@talbs , help, what's the right class for the note? https://github.com/edx/edx-platform/blob/22f1640ca12c4e99cd936420a84c895ab79f3304/lms/templates/combinedopenended/combined_open_ended.html#L62-L67

@waheedahmed okay?