docs/sys: Detailed description of print_exception() diff from traceback module.

docs/library/sys.rst

@@ -21,7 +21,12 @@ Functions
    .. admonition:: Difference to CPython
       :class: attention
 
-      This function appears in the ``traceback`` module in CPython.
+      This is simplified version of a function which appears in the
+      ``traceback`` module in CPython. Unlike ``traceback.print_exception()``,
+      this function takes just exception value instead of exception type,
+      exception value, and traceback object; `file` argument should be
+      positional; further arguments are not support. Compliant
+      ``traceback`` module can be found in micropython-lib.
 
 Constants
 ---------