Bug 18636: Sysprefs: Add explanation for conflict autonumbernum / BorrowerMandatoryFields

Bug 18636: Sysprefs: Add explanation for conflict autonumbernum / BorrowerMandatoryFields

This patch adds a note to the system preferences autonembernum and
BorrowerMandatoryFields regarding a conflict if automembernum is on
and BorrowerMandatoryFields contains cardnumber.

To reproduce issue: See initial comment.

To test:
- Apply patch
- Verify that in system preferences note appears with both prefs
  automembernum and BorrowerMandatoryFields

Followed test plan, works as described
Signed-off-by: Alex Buckley <alexbuckley@catalyst.net.nz>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>