pep-550: Add a return (#346)

* pep-550: Add a return

* pep-550: Clarify that ContextItem.get() cache uses a borrowed ref

pep-0550.rst

@@ -222,6 +222,8 @@ follows (in pseudo-code)::
                 if self in local_context:
                     return local_context[self]
 
+            return None
+
         def set(self, value):
             tstate = PyThreadState_Get()
 
@@ -838,13 +840,17 @@ The above two fields allow implementing a fast cache path in
                     value = mapping[self]
                     break
 
-            self.last_value = value
+            self.last_value = value  # borrowed ref
             self.last_tstate_id = tstate.unique_id
             self.last_ver = tstate.execution_context_ver
             self.last_deallocs = tstate.interp.context_item_deallocs
 
             return value
 
+Note that ``last_value`` is a borrowed reference.  The assumption
+is that if all counters tests are OK, the object will be alive.
+This allows the CI values to be properly GCed.
+
 This is similar to the trick that decimal C implementation uses
 for caching the current decimal context, and will have the same
 performance characteristics, but available to all