gh-137871: Clarify cmath.nan documentation by linking to math module (GH-137876)

(cherry picked from commit 025b403)

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

Author: 2 people

Doc/library/cmath.rst

@@ -338,7 +338,7 @@ Constants
 .. data:: nan
 
    A floating-point "not a number" (NaN) value.  Equivalent to
-   ``float('nan')``.
+   ``float('nan')``. See also :data:`math.nan`.
 
    .. versionadded:: 3.6