update changelog

Author: bagerard

docs/changelog.rst

@@ -12,6 +12,7 @@ Development
     - make sure to read https://www.mongodb.com/docs/manual/core/transactions-in-applications/#callback-api-vs-core-api
     - run_in_transaction context manager relies on Pymongo coreAPI, it will retry automatically in case of `UnknownTransactionCommitResult` but not `TransientTransactionError` exceptions
     - Using .count() in a transaction will always use Collection.count_document (as estimated_document_count is not supported in transactions)
+- Fix use of $geoNear or $collStats in aggregate #2493
 
 Changes in 0.29.0
 =================