expand open_service_nursery() example

tricycle/_service_nursery.py

@@ -68,8 +68,26 @@ async def open_service_nursery() -> AsyncIterator[trio.Nursery]:
     ``conn`` to remain usable in any ``finally`` or ``__aexit__``
     blocks in ``communicate_with_websocket()``.  With a regular
     nursery, this is not guaranteed; with a service nursery, it is.
-
-    Child tasks spawned using ``start()`` gain their protection from
+    An example hinting at general usage::
+
+        @asynccontextmanager
+        async def open_websocket(host, port):
+            async with open_service_nursery() as nursery:
+                try:
+                    # ... make some child tasks ...
+                    yield connection
+                finally:
+                    # The yield body is already cancelled, and
+                    # child tasks are still available here for cleanup...
+                    pass
+
+    Now, anything in the body of the ``open_websocket()`` context, including
+    ``communicate_with_websocket()``, will be given first opportunity to cancel
+    gracefully.  Subsequently, the ``finally`` block in the ``open_websocket()``
+    implementation runs, and tasks spawned within the ``try`` body are still
+    available during cleanup.
+
+    Note that child tasks spawned using ``start()`` gain their protection from
     premature cancellation only at the point of their call to
     ``task_status.started()``.
     """