Update website/www/site/content/en/documentation/programming-guide.md

Co-authored-by: tvalentyn <[email protected]>

website/www/site/content/en/documentation/programming-guide.md

@@ -1218,7 +1218,7 @@ Here is a sequence diagram that shows the lifecycle of the DoFn during
  isn't guaranteed.
  2. the number of DoFn instances created at runtime is runner-dependent.
  3. for the Python SDK, the pipeline contents such as DoFn user code,
- is [serialized into a bytecode](https://beam.apache.org/documentation/sdks/python-pipeline-dependencies/#pickling-and-managing-the-main-session). Use [this module](https://beam.apache.org/releases/pydoc/current/apache_beam.utils.shared.html) to manage a single instance of a DoFn shared by multiple threads within the same process.
+ is [serialized into a bytecode](https://beam.apache.org/documentation/sdks/python-pipeline-dependencies/#pickling-and-managing-the-main-session). Therefore, `DoFn`s should not reference objects that are not serializable, such as locks. To manage a single instance of an object across multiple `DoFn` instances in the same process, use utilities in the [shared.py](https://beam.apache.org/releases/pydoc/current/apache_beam.utils.shared.html) module.
 
 <!-- The source for the sequence diagram can be found in the SVG resource. -->
 ![This is a sequence diagram that shows the lifecycle of the DoFn](/images/dofn-sequence-diagram.svg)