Add sub-orchestration documentation

Author: andystaples

docs/features.md

@@ -20,7 +20,7 @@ Orchestrations can schedule durable timers using the `create_timer` API. These t
 
 ### Sub-orchestrations
 
-Orchestrations can start child orchestrations using the `call_sub_orchestrator` API. Child orchestrations are useful for encapsulating complex logic and for breaking up large orchestrations into smaller, more manageable pieces.
+Orchestrations can start child orchestrations using the `call_sub_orchestrator` API. Child orchestrations are useful for encapsulating complex logic and for breaking up large orchestrations into smaller, more manageable pieces. Sub-orchestrations can also be versioned in a similar manner to their parent orchestrations, however, they do not inherit the parent's version. Instead, they will use the default_version defined in the current worker's VersioningOptions unless otherwise specified during `call_sub_orchestrator`.
 
 ### External events
 

durabletask/worker.py

@@ -111,7 +111,7 @@ def __init__(self, version: Optional[str] = None,
 
         Args:
             version: The version of orchestrations that the worker can work on.
-            default_version: The default version that will be used for starting new orchestrations.
+            default_version: The default version that will be used for starting new sub-orchestrations.
             match_strategy: The versioning strategy for the Durable Task worker.
             failure_strategy: The versioning failure strategy for the Durable Task worker.
         """