DOC: add additional qualifier to expected behavior of PyOS_InputHook

Author: tacaswell

Doc/c-api/veryhigh.rst

@@ -151,7 +151,8 @@ the same library that the Python runtime is using.
    event loops, as done in :file:`Modules/_tkinter.c` in the
    Python source code.
 
-   This function should block until stdin is readable.
+   This function should block until stdin is readable but may return
+   early.
 
    .. versionchanged:: 3.12
       This function is only called from the