micropython.const: Improve docstring, add example.

This merges the useful information from two different places:
- The original docstring for const.
- Writing MicroPython code for constrained devices.

Also reduce type to just int, because only ints are supported.

examples/micropython/const.py

@@ -0,0 +1,13 @@
+from micropython import const
+
+# This value can be used here. Other files can import it too.
+APPLES = const(123)
+
+# These values can only be used within this file.
+_ORANGES = const(1 << 8)
+_BANANAS = const(789 + _ORANGES)
+
+# You can read the constants as normal values. The compiler
+# will just insert the numeric values for you.
+fruit = APPLES + _ORANGES + _BANANAS
+print(fruit)