Bug 12802: Add note to EmailFieldPrimary system preference

Bug 12802: Add note to EmailFieldPrimary system preference

To clarify that "first valid" should be unchecked for multiple emails to receive notices

Signed-off-by: Brendan Gallagher <brendan@bywatersolutions.com>
Signed-off-by: Axelle Clarisse <axelle.clarisse@univ-amu.fr>
Signed-off-by: Mathieu Saby <mathsabypro@gmail.com>
Signed-off-by: Aleisha Amohia <aleisha@catalyst.net.nz>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>