Clarify "at most once" guarantee is per-invocation of lookup-or-miss

Author: seancorfield

src/main/clojure/clojure/core/cache/wrapped.clj

@@ -40,6 +40,7 @@
   value (using value-fn, and optionally wrap-fn), update the cache for `e`
   and then perform the lookup again.
 
+  For any given invocation of lookup-or-miss, it is guaranteed that
   value-fn (and wrap-fn) will only be called (at most) once even in the
   case of retries, so there is no risk of cache stampede.