MONGOID-4956 add query cache information to release notes (#4889)

Author: Emily Giurleo

source/tutorials/mongoid-upgrade.txt

@@ -362,6 +362,42 @@ Example Mongoid 7.1 behavior:
 Mongoid provides :ref:`sharding management Rake tasks <sharding-management>`
 to shard collections according to shard keys declared in models.
 
+Query Cache Moved to Driver
+---------------------------
+
+Minor change: Ruby driver version 2.14.0 implements a new and improved query
+cache. When using driver version 2.14.0 or newer, Mongoid will use the driver's
+query cache to cache query results.
+
+The driver query cache introduces the following improvements:
+
+* Caching multi-batch query results
+* Taking a query's read concern and read preference into account when deciding
+  when to return cached results
+* Invalidating the cache after bulk write operations and aggregation operations
+  with ``$out`` and ``$merge`` pipeline stages
+* Invalidating the cache after transaction commit and abort operations
+* Improved performance of queries with limits
+* Caching aggregation results
+* More efficient query cache invalidation
+
+Mongoid's query cache, which will now be referred to as the "legacy query cache,"
+has been deprecated. Mongoid will retain the legacy query cache for use with
+older versions of the driver.
+
+The interface for enabling and disabling the query cache in Mongoid has not
+changed. When using driver versions 2.14.0 and newer, this interface will
+enable or disable the query cache in the driver.
+
+The driver query cache is more correct and more effective than the legacy
+Mongoid query cache. If you plan to use the query cache, it is recommended
+that you upgrade to driver version 2.14.
+
+To read more about the query cache improvements made in the driver, see
+`the Ruby driver documentation <https://docs.mongodb.com/ruby-driver/current/tutorials/query-cache/>`_.
+
+To read more about using the query cache with Mongoid and the limitations
+of the legacy query cache, see :ref:`the query cache documentation <query-cache>`.
 
 Upgrading to Mongoid 7.1
 ========================