docs/library: Note link between machine.soft_reset() and sys.exit().

This is currently an implementation detail of MicroPython rather than by
design.

This work was funded through GitHub Sponsors.

Signed-off-by: Angus Gratton <angus@redyak.com.au>

docs/library/builtins.rst

@@ -194,6 +194,12 @@ Exceptions
 
     |see_cpython| `python:SystemExit`.
 
+    On non-embedded ports (i.e. Windows and Unix), an unhandled ``SystemExit``
+    exits the MicroPython process in a similar way to CPython.
+
+    On embedded ports, an unhandled ``SystemExit`` currently causes a
+    :ref:`soft_reset` of MicroPython.
+
 .. exception:: TypeError
 
     |see_cpython| `python:TypeError`.

docs/library/sys.rst

@@ -12,9 +12,12 @@ Functions
 .. function:: exit(retval=0, /)
 
    Terminate current program with a given exit code. Underlyingly, this
-   function raise as `SystemExit` exception. If an argument is given, its
+   function raises a `SystemExit` exception. If an argument is given, its
    value given as an argument to `SystemExit`.
 
+   On embedded ports (i.e. all ports but Windows and Unix), an unhandled
+   `SystemExit` currently causes a :ref:`soft_reset` of MicroPython.
+
 .. function:: atexit(func)
 
    Register *func* to be called upon termination.  *func* must be a callable