fix swagger documentation for multiple files API endpoint (#25110)

fix swagger documentation for multiple files API endpoint (#25110)

Fixes some issues with the swagger documentation for the new multiple
files API endpoint (#24887) which were overlooked when submitting the
original PR:

1. add some missing parameter descriptions
2. set correct `required` option for required parameters
3. change endpoint description to match it full functionality (every
kind of file modification is supported, not just creating and updating)