TW-1587: Further improve example on-exit hook

- Initial patch was not quite right.  on-exit _does_ receive added and
  modified tasks on STDIN, but is not allowed to return JSON.

scripts/hooks/on-exit

@@ -5,9 +5,16 @@
 
 # Output:
 # - Optional feedback/error.
-echo 'on-exit'
+
+n=0
+while read modified_task
+do
+  n=$(($n + 1))
+done
+
+echo "on-exit: Counted $n added/modified tasks."
 
 # Status:
-# - 0:     JSON ignored, non-JSON is feedback.
-# - non-0: JSON ignored, non-JSON is error.
+# - 0:     Non-JSON is feedback.
+# - non-0: Non-JSON is error.
 exit 0