New documentation available at https://docs.openkm.com/kcenter/
The documentation at the https://www.openkm.com/wiki is definitivily deprecated.
Consider only use it, if you are user of OpenKM version 6.2 and older.
Click in the banner to close the warning.
Select some filebrowser rows.
Execute the action add note will be shown a popup.
Write the note and Click Add button. The note will be added in each selected rows objects.