docs: explain diff.*.binary option

docs: explain diff.*.binary option

This was added long ago as part of the userdiff refactoring
for textconv, as internally it made the code simpler and
cleaner. However, there was never a concrete use case for
actually using the config variable.

Now that Matthieu Moy has provided such a use case, it's
easy to explain it using his example.

Signed-off-by: Jeff King <peff@peff.net>
Acked-by: Matthieu Moy <Matthieu.Moy@grenoble-inp.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>