Describe optional taskset field in the task context docs.

docs/userguide/tasks.rst

@@ -49,6 +49,9 @@ attributes:
 
 :id: The unique id of the executing task.
 
+:taskset: The unique id of the taskset that owns the executing task.
+          This key exists only if the task is a member of a taskset.
+
 :args: Positional arguments.
 
 :kwargs: Keyword arguments.