docs/library/sys.rst: Add note about '.frozen' as an entry in sys.path.

Signed-off-by: Jim Mussared <jim.mussared@gmail.com>

docs/library/sys.rst

@@ -115,6 +115,14 @@ Constants
 
    A mutable list of directories to search for imported modules.
 
+   .. admonition:: Difference to CPython
+      :class: attention
+
+      On MicroPython, an entry with the value ``".frozen"`` will indicate that import
+      should search :term:`frozen modules <frozen module>` at that point in the search.
+      If no frozen module is found then search will *not* look for a directory called
+      ``.frozen``, instead it will continue with the next entry in ``sys.path``.
+
 .. data:: platform
 
    The platform that MicroPython is running on. For OS/RTOS ports, this is