Bug 32964: OPACResultsMaxItemsUnavailable description is misleading

Bug 32964: OPACResultsMaxItemsUnavailable description is misleading

This patch rephrases the last part of the description of the
OPACResultsMaxItemsUnavailable system preference to clarify the link
to the OPACResultsUnavailableGroupingBy option.

It also changes some terminology to follow the guidelines
(capitalization, library instead of branch).

To test:
1. Apply patch
2. Go to Administration > Global system preferences
3. Search for OPACResultsMaxItems
4. Read the sentence for OPACResultsMaxItemsUnavailable, make sure it
makes sense, the grammar and spelling are ok, respects terminology
and capitalization guidelines
5. Try the link in the description, make sure it goes to the correct
syspref
6. Read the sentence and options for OPACResultsUnavailableGroupingBy,
make sure it makes sense, the grammar and spelling are ok, respects
terminology and capitalization guidelines
7. Optionally, try the various options, they should still work as
intended

Signed-off-by: Laura Escamilla <laura.escamilla@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>