docs: math, cmath: Add port availability information.

docs/library/cmath.rst

@@ -4,9 +4,12 @@
 .. module:: cmath
    :synopsis: mathematical functions for complex numbers
 
-The ``cmath`` module provides some basic mathematical funtions for
+The ``cmath`` module provides some basic mathematical functions for
 working with complex numbers.
 
+Availability: not available on WiPy and ESP8266. Floating point support
+required for this module.
+
 Functions
 ---------
 

docs/library/math.rst

@@ -9,6 +9,9 @@ working with floating-point numbers.
 
 *Note:* On the pyboard, floating-point numbers have 32-bit precision.
 
+Availability: not available on WiPy. Floating point support required
+for this module.
+
 Functions
 ---------