*
* This instantiates a hold-off period where the key cannot be
* written to avoid race conditions where dependent keys get updated
- * with a stale value (e.g. from a DB slave).
+ * with a stale value (e.g. from a DB slave). This is implemented by
+ * storing a special "tombstone" value at the cache key that this
+ * class recognizes; get() calls will return false for the key.
*
* This should only be called when the underlying data (being cached)
* changes in a significant way. If called twice on the same key, then