update changelog

Author: d-v-b

changes/3547.misc.md

@@ -1,2 +1,4 @@
 Moved concurrency-limiting functionality to store classes. The global configuration object no longer
-controls concurrency limits. Concurrency limits, if applicable, must now be specified when constructing a store.
+controls concurrency limits. Concurrency limits, if applicable, must now be specified when constructing a store.
+Users who have subclassed the `Store` class should implement concurrency-limiting logic if appropriate for
+their storage backend.