Describe perf benefit of reusing iterator from same version

include/VersionedMap.h

@@ -136,7 +136,9 @@ struct VersionedMap {
   /** Perform `count` "first greater than or equal to" queries. The result of
    * querying `key[i]` at `version[i]` is `iterator[i]`. `version[i]` must be >=
    * `getOldestVersion()` and <= `getVersion()`. Thread-safe as long as a
-   * version is not concurrently invalidated by `setOldestVersion`. */
+   * version is not concurrently invalidated by `setOldestVersion`. There's a
+   * performance benefit if you pass iterator[i] previously obtained at
+   * version[i]. */
   void firstGeq(const Key *key, const int64_t *version, Iterator *iterator,
                 int count) const;