Bug 23751: Make staffaccess permission description more precise

Bug 23751: Make staffaccess permission description more precise

The permission description was missing 'username' from the description.

To test:
- Look at the permissions of a patron
- Verify the staffaccess permission is labelled:
  Allow staff members to modify permissions and passwords for other staff members
- Apply patch
- Check description again:
  Allow staff members to modify permissions, usernames, and passwords for other staff members

Signed-off-by: Séverine QUEUNE <severine.queune@bulac.fr>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>