Add a comment explaining why _handle_job() is a separate method

Author: cslecrone

trio/_core/_thread_cache.py

@@ -57,6 +57,8 @@ def __init__(self, thread_cache):
         thread.start()
 
     def _handle_job(self):
+        # Handle job in a separate method to ensure user-created
+        # objects are cleaned up in a consistent manner. 
         fn, deliver = self._job
         self._job = None
         result = outcome.capture(fn)