ui: calendar event: add once daily example and clarify workday one

ui: calendar event: add once daily example and clarify workday one

similar to PVE, copying over the remaining commit message:

Using 00:00 with relying on the implied default is sub optimal as its
a bit of a magic example that new users may not understand as easily.
So spell it out explicitly, even if there'd be a shorter version
possible.

We also had some request for the once-daily every day, and its a
sensible example to have in general, could help getting the
difference between an hour list and a single one.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>