Clarify why bug12985 is a thing

changes/bug12985

@@ -1,4 +1,5 @@
   o Minor bugfixes (shutdown):
     - When shutting down, always call event_del() on lingering read or
-      write events before freeing them. Fixes bug 12985; bugfix on
+      write events before freeing them. Otherwise, we risk double-frees
+      or read-after-frees in event_base_free(). Fixes bug 12985; bugfix on
       0.1.0.2-rc.