docs(invoke): explain why error.execution uses the external queue

Invoke handlers run in background threads, outside the processing loop.
Using the internal queue would either contaminate an unrelated macrostep
or stall indefinitely. This matches done.invoke, which also uses the
external queue for the same reason.

Author: fgmacedo

statemachine/invoke.py

@@ -391,6 +391,12 @@ def _run_sync_handler(
                 )
         except Exception as e:
             if not ctx.cancelled.is_set():
+                # Intentionally using the external queue (no internal=True):
+                # This handler runs in a background thread, outside the processing
+                # loop. Using the internal queue would either contaminate an
+                # unrelated macrostep in progress, or stall if no macrostep is
+                # active (the internal queue is only drained within a macrostep).
+                # This matches done.invoke, which also uses the external queue.
                 self.sm.send("error.execution", error=e)
         finally:
             invocation.terminated = True
@@ -459,6 +465,7 @@ async def _run_async_handler(
             return
         except Exception as e:
             if not ctx.cancelled.is_set():
+                # External queue — see comment in _run_sync_handler.
                 self.sm.send("error.execution", error=e)
         finally:
             invocation.terminated = True