Describe a potential (rare) distringuisher in idle circuits.

In the rare event that a user resumes activity after a period between the
"reduced connection timeout" and the full value, and that user has not set
reduced padding, this is a distinguisher on circuits that have been held idle
and open for that long.

padding-spec.txt

@@ -306,6 +306,9 @@ Table of Contents
         serves as a reasonable default regardless of implementation.
       - For all use cases, reduced padding clients use half the consensus
         value.
+      - Implementations MAY mark circuits held open past the reduced padding
+        quantity (half the consensus value) as "not to be used for streams",
+        to prevent their use from becoming a distinguisher.
       - Default: 1800
 
     * nf_pad_before_usage