Docs: Clarify cmath.nan/inf by linking to math module

The documentation for `cmath.nan` and `cmath.inf` is updated to
cross-reference the equivalent constants in the `math` module.

The `math` module's documentation contains a more thorough
explanation of the properties of these special floating-point
values, as required by the IEEE 754 standard (e.g., that NaN
does not compare equal to itself).

Author: AzizCode92

Doc/library/cmath.rst

@@ -322,7 +322,7 @@ Constants
 
 .. data:: inf
 
-   Floating-point positive infinity. Equivalent to ``float('inf')``.
+   Floating-point positive infinity. Equivalent to :data:`math.inf`.
 
    .. versionadded:: 3.6
 
@@ -338,7 +338,7 @@ Constants
 .. data:: nan
 
    A floating-point "not a number" (NaN) value.  Equivalent to
-   ``float('nan')``.
+   :data:`math.nan`.
 
    .. versionadded:: 3.6