docs(run[deadline]) Note callback fires on stderr only

why: ``_wait_with_deadline`` now drains both ``stdout`` and ``stderr``
to prevent a chatty child from deadlocking on a full pipe, but the
progress callback only fires for ``stderr`` chunks (matching the legacy
non-timeout codepath). Without an explicit note, callers who pass
``log_in_real_time=True`` and expect ``stdout`` lines to surface in
their callback will be surprised when the buffer is silent.
what:
- Add a Notes paragraph to ``_wait_with_deadline``'s docstring stating
  that the callback fires on ``stderr`` chunks only by design, and
  pointing callers who want streaming ``stdout`` toward redirecting
  the stream themselves instead of relying on the callback.

Author: tony

src/libvcs/_internal/run.py

@@ -342,6 +342,15 @@ def _wait_with_deadline(
         ``None`` when the corresponding pipe could not be put into non-
         blocking mode (Windows pipes, unusual fd types) -- in that case the
         caller should fall back to reading the pipe directly.
+
+    Notes
+    -----
+    The progress ``callback`` is invoked for ``stderr`` chunks only,
+    matching the legacy non-timeout codepath. ``stdout`` is drained into
+    the returned buffer to prevent the child from blocking on a full pipe,
+    but its chunks are not forwarded to the callback in real time. Callers
+    that want streaming ``stdout`` should redirect it themselves
+    (e.g. ``stdout=`` to a file) rather than relying on ``callback``.
     """
     sel = selectors.DefaultSelector()
     buffers: dict[t.IO[bytes], list[bytes]] = {}