Much of the ArmA mission making is same as in OFP, so you should refer to the OFP Missions area for details. However here are the ArmA specific stuff.
The default BIS class names for all units and vehicles.
All default music tracks listed.
List of EventHandler's.
How does the teamswitch work, what commands are available.
Most likely in the future all our mission editing additions will be placed only in ArmA area for obvious reasons.