bpo-35461: Document C API functions which suppress exceptions. (GH-11119)

diff --git a/Doc/c-api/number.rst b/Doc/c-api/number.rst
index 3c7605a..296b21c 100644
--- a/Doc/c-api/number.rst
+++ b/Doc/c-api/number.rst
@@ -280,3 +280,4 @@
 
    Returns ``1`` if *o* is an index integer (has the nb_index slot of  the
    tp_as_number structure filled in), and ``0`` otherwise.
+   This function always succeeds.