chore: fix typing exports, add operator docstring

changes.md

@@ -2,6 +2,9 @@
 
 ## Unreleased
 
+- Typing: Added `Action` and `StartableFactory` to `reactivex.typing.__all__`,
+  making them part of the explicit public API surface.
+- Docs: Added missing docstring to `on_error_resume_next` operator.
 - Operators: Fixed scheduler forwarding in `pairwise`, `to_marbles`, and
   `delay_with_mapper` (subscription-delay path). These operators now pass the
   `scheduler` argument through to `source.subscribe(...)` and, in the case of

reactivex/operators/_onerrorresumenext.py

@@ -11,6 +11,16 @@ def on_error_resume_next_(
     second: Observable[_T],
 ) -> Callable[[Observable[_T]], Observable[_T]]:
     def on_error_resume_next(source: Observable[_T]) -> Observable[_T]:
+        """Continues an observable sequence that is terminated normally or
+        by an exception with the next observable sequence.
+
+        Args:
+            source: First observable sequence.
+
+        Returns:
+            An observable sequence that concatenates the source and second
+            sequences, even if the source sequence terminates exceptionally.
+        """
         return reactivex.on_error_resume_next(source, second)
 
     return on_error_resume_next

reactivex/typing.py

@@ -49,6 +49,7 @@
     "Accumulator",
     "AbsoluteTime",
     "AbsoluteOrRelativeTime",
+    "Action",
     "Comparer",
     "Mapper",
     "MapperIndexed",
@@ -63,6 +64,7 @@
     "ScheduledSingleOrPeriodicAction",
     "ScheduledAction",
     "Startable",
+    "StartableFactory",
     "StartableTarget",
     "Subscription",
 ]