Bug 27775: Add hint about drag and drop feature on framework subfield edit

Bug 27775: Add hint about drag and drop feature on framework subfield edit

Bug 8976 introduced a way to sort subfields on a MARC/Auth framework using drag and drop. But there's no visible hint alerting the user the feature exists.

Test plan :
1) Go to Admistration > MARC bibliographic framework
2) Choose a framework and click on Actions > MARC structure
3) Choose a field and click on Actions > Edit subfields
4) Check you see the hint
5) Go to Admistration > Authority types
6) Choose a type and click on Actions > MARC structure
7) Choose a field and click on Actions > Subfields
8) Click on Edit subfields
9) Check you see the hint

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>