docs: add docstring to contextmanager_in_threadpool

Add missing docstring to the contextmanager_in_threadpool utility
function in fastapi/concurrency.py.

fastapi/concurrency.py

@@ -16,6 +16,14 @@ _T = TypeVar("_T")
 
 @asynccontextmanager
 async def contextmanager_in_threadpool(
+    """Run a context manager in a thread pool.
+    
+        Args:
+            cm: The context manager to run.
+    
+        Returns:
+            The result of the context manager.
+        """
     cm: AbstractContextManager[_T],
 ) -> AsyncGenerator[_T, None]:
     # blocking __exit__ from running waiting on a free thread