docs(example): note null→0 coercion in above-threshold UDF

The `(v.as_py() or 0) > threshold` line was reading as canonical Python
UDF style. Add a one-line comment flagging that nulls silently coerce
to 0 — fine for the demo dataset (no nulls), but real code should pick
the semantics explicitly.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: timsaucer

examples/multiprocessing_pickle_expr.py

@@ -64,6 +64,9 @@ def make_above_threshold_udf(threshold: int) -> ScalarUDF:
     """
 
     def above(arr: pa.Array) -> pa.Array:
+        # `v.as_py() or 0` coerces nulls to 0 — the demo dataset has no
+        # nulls, but real-world code should decide explicitly how nulls
+        # compare against the threshold.
         return pa.array([1 if (v.as_py() or 0) > threshold else 0 for v in arr])
 
     return udf(