Bug 20057: Fix capitalization and rephrase system preference as a sentence

Bug 20057: Fix capitalization and rephrase system preference as a sentence

This makes changes to strings involved in this feature:
- Rephrases the system preference description into a sentence
- Rephrases message shown in OPAC on submitting (library has not actively
  approved, so just say that the record has been modified)

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>