Bug 10174: Add a tooltip to explain what is a digest

Bug 10174: Add a tooltip to explain what is a digest

The digest term seems to be ambiguous for some people.

This patch adds a tooltip to explain what it is (feel free to provide
a better wording).

Test plan:
Go at the OPAC and click on the "Your messaging tab", you should see an
icon close to "Digest only" to explain what is a digest.
Same at the intranet on editing/showing patron info

Signed-off-by: Nick Clemens <nick@quecheelibrary.org>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>
(cherry picked from commit d2dd4ca624aba7e93baab02c8efdc8a29a15a74e)
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>