[3.13] pythongh-141004: Document Py_RETURN_NAN and Py_RETURN_INF (pythonGH-141029) (pythonGH-141075)

pythongh-141004: Document `Py_RETURN_NAN` and `Py_RETURN_INF` (pythonGH-141029)
(cherry picked from commit 5b02c6e)

Co-authored-by: Peter Bierma <[email protected]>
Co-authored-by: Sergey B Kirpichev <[email protected]>

Author: 3 people

Doc/c-api/float.rst

@@ -78,6 +78,23 @@ Floating-Point Objects
    Return the minimum normalized positive float *DBL_MIN* as C :c:expr:`double`.
 
 
+.. c:macro:: Py_RETURN_NAN
+
+   Return :data:`math.nan` from a function.
+
+   On most platforms, this is equivalent to ``return PyFloat_FromDouble(NAN)``.
+
+
+.. c:macro:: Py_RETURN_INF(sign)
+
+   Return :data:`math.inf` or :data:`-math.inf <math.inf>` from a function,
+   depending on the sign of *sign*.
+
+   On most platforms, this is equivalent to the following::
+
+      return PyFloat_FromDouble(copysign(INFINITY, sign));
+
+
 Pack and Unpack functions
 -------------------------