docs: Add CPy diff note for print_exception; embellish sys.platform.

docs/library/sys.rst

@@ -17,6 +17,10 @@ Functions
    Print exception with a traceback to a file-like object ``file`` (or
    ``sys.stdout`` by default).
 
+   .. admonition:: Difference to CPython
+
+      This function appears in the ``traceback`` module in CPython.
+
 Constants
 ---------
 
@@ -34,7 +38,9 @@ Constants
 
 .. data:: platform
 
-   the platform that Micro Python is running on
+   The platform that Micro Python is running on.  This is "pyboard" on the
+   pyboard and provides a robust way of determining if a script is running
+   on the pyboard or not.
 
 .. data:: stderr