add missing calendar-syntax doc

Syntax documentation is listed in the documentation content but not present

this is a starting point based on what I found, but I missed some points

- how to create a few days event (example: from 2014-10-10 to 2014-10-12), and is it possible to specified starting and finishing hours for this kind of events?
- is it possible to add notes to event?

extra question:
- is it possible to see event notes directly in the vim interface?

btw, thanks a lot this is a realy good plugin ;-)