docs(expr): restate to_bytes behavior when ctx is None

The previous docstring on Expr.to_bytes called out the no-ctx case
("Without ctx a default codec is used"). The pickle-support rewrite
replaced the docstring with a description of what travels inside the
bytes but dropped the sentence that answered the immediate question
a reader has when they see the optional `ctx` parameter.

Re-add a one-paragraph explanation: with `ctx`, encoding routes
through the session's installed LogicalExtensionCodec — so
`with_python_udf_inlining` takes effect; without `ctx`, the default
codec is used (Python UDF inlining on, no user-installed extension
codec).

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

Author: timsaucer

python/datafusion/expr.py

@@ -439,6 +439,12 @@ def to_bytes(self, ctx: SessionContext | None = None) -> bytes:
         Use this — or :func:`pickle.dumps` — to send an expression to a
         worker process for distributed evaluation.
 
+        When ``ctx`` is supplied, encoding routes through that session's
+        installed :class:`LogicalExtensionCodec` (so settings like
+        :meth:`SessionContext.with_python_udf_inlining` take effect).
+        When ``ctx`` is ``None``, the default codec is used (Python UDF
+        inlining on, no user-installed extension codec).
+
         Built-in functions and Python UDFs (scalar, aggregate, window)
         travel inside the returned bytes; the worker does not need to
         pre-register them. UDFs imported via the FFI capsule protocol