Issue #13522: document error return values of some float and complex C API functions.
diff --git a/Doc/c-api/float.rst b/Doc/c-api/float.rst
index 757efd3..27a75e3 100644
--- a/Doc/c-api/float.rst
+++ b/Doc/c-api/float.rst
@@ -47,6 +47,8 @@
Return a C :c:type:`double` representation of the contents of *pyfloat*. If
*pyfloat* is not a Python floating point object but has a :meth:`__float__`
method, this method will first be called to convert *pyfloat* into a float.
+ This method returns ``-1.0`` upon failure, so one should call
+ :c:func:`PyErr_Occurred` to check for errors.
.. c:function:: double PyFloat_AS_DOUBLE(PyObject *pyfloat)