PYTHON-5212 changelog update (#2306)

Author: NoahStapp

doc/changelog.rst

@@ -16,6 +16,8 @@ Version 4.12.1 is a bug fix release.
   errors such as: "NotImplementedError: Database objects do not implement truth value testing or bool()".
 - Removed Eventlet testing against Python versions newer than 3.9 since
   Eventlet is actively being sunset by its maintainers and has compatibility issues with PyMongo's dnspython dependency.
+- Fixed a bug where MongoDB cluster topology changes could cause asynchronous operations to take much longer to complete
+  due to holding the Topology lock while closing stale connections.
 
 Issues Resolved
 ...............