add changelog

Author: toshok

CHANGELOG.md

@@ -42,6 +42,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   ([#3882](https://github.com/open-telemetry/opentelemetry-python-contrib/pull/3882))
 - `opentelemetry-instrumentation-aiohttp-server`: delay initialization of tracer, meter and excluded urls to instrumentation for testability
   ([#3836](https://github.com/open-telemetry/opentelemetry-python-contrib/pull/3836))
+- `opentelemetry-instrumentation-pymongo`: Fix span error descriptions
+  ([#3904](https://github.com/open-telemetry/opentelemetry-python-contrib/pull/3904))
 
 ## Version 1.38.0/0.59b0 (2025-10-16)