The Business Process Management guide documents only basic BPMN2 constructs and has not changed for a very long time. Now both the engine and the web Designer support more of them. Here is the list of BPMN2 constructs which should be documented (some of them are already, but sometimes badly documented):

Start events:
Start Message Event
Start Timer Event
Start Escalation Event
Start Event
Start Conditional Event
Start Error Event
Start Compensation Event
Start Signal Event

Catching intermediate events:
Conditional Intermediate Event
Error Intermediate Event
Escalation Intermediate Event
Message Intermediate Event
Signal Intermediate Event
Timer Intermediate Event
Throwing intermediate events:
Escalation Intermediate Event
Message Intermediate Event
Signal Intermediate Event

End events:
Cancel End Event
Compensation End Event
End Event
Error End Event
Escalation End Event
Message End Event
Signal End Event
Terminate End Event

Few of these events are documented, the description is often deprecated or might miss important information. For example the error handling mechanism of the error events should be described more clearly.

Activities:
Send Task
Receive Task
Manual Task
AdHoc Subprocess

Message sending should be described and also the difference between using of Send/Receive Task and Message Events.

Gateways:
Inclusive Gateway
Event-based Gateway

And maybe other features like:
Swimlanes
Data Objects
Connecting Objects
Groups and Annotations
...