[3.14] pythongh-137871: Clarify cmath.nan documentation by linking to math module (pythonGH-137876) (python#140095)

Co-authored-by: Aziz <azizbenothman76@gmail.com>
Co-authored-by: Sergey B Kirpichev <skirpichev@gmail.com>

Author: 3 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