gh-156662: Note that TaskGroup.create_task() raises RuntimeError (#156663)

Doc/library/asyncio-task.rst

diff --git a/Doc/library/asyncio-task.rst b/Doc/library/asyncio-task.rst
index 689a6a6..1a13ad3 100644
--- a/Doc/library/asyncio-task.rst
+++ b/Doc/library/asyncio-task.rst
@@ -356,7 +356,7 @@
       The signature matches that of :func:`asyncio.create_task`.
       If the task group is inactive (e.g. not yet entered,
       already finished, or in the process of shutting down),
-      we will close the given ``coro``.
+      we will close the given ``coro`` and raise :exc:`RuntimeError`.
 
       .. versionchanged:: 3.13