docs/library: Clarify relation between machine and port-specific mods.
@@ -117,6 +117,18 @@ the following libraries.
uctypes.rst
+Port-specific libraries
+-----------------------
+
+In some cases the following port/board-specific libraries have functions or
+classes similar to those in the :mod:`machine` library. Where this occurs, the
+entry in the port specific library exposes hardware functionality unique to
+that platform.
+
+To write portable code use functions and classes from the :mod:`machine` module.
+To access platform-specific hardware use the appropriate library, e.g.
+:mod:`pyb` in the case of the Pyboard.
+
Libraries specific to the Unix port
-----------------------------------
