Bug 34240: Added hint about having to use Koha-to-MARC mappings for Koha link in...

Bug 34240: Added hint about having to use Koha-to-MARC mappings for Koha link in frameworks

To test:
1. Go to administration > marc bibliographic framework > choose framework > marc structure
choose any tag
2. go to actions and edit subfields
3. check that the koha link pulldown can not be edited
=> There is no hint
4. Apply patch
5. Refresh the page, the select box is replaced by text and there is a hint on how to edit

Signed-off-by: Christina Fairlamb <cjf@wmu.se>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>