Bug 15233: Cataloging subfield editors: Clean up html and streamline text for better...

 Bug 15233: Cataloging subfield editors: Clean up html and streamline text for better translatability

This patch removes bold markup from explanatory text in auth_subfields_structure.tt
and makes it the same as in marc_subfields_structure.tt for better translatibility.

To test:
- Apply patch
- Verify that string changes make sense and that the explanatory text is the same in both files.

Signed-off-by: Hector Castro <hector.hecaxmmx@gmail.com>
Rewording Ok for translation purposes

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Brendan A Gallagher <brendan@bywatersolutions.com>