clarify that time.perf_counter cannot go backwards

I'm not completely sure about this, but it seems to me this counter is *also* designed to avoid jumping backwards on (say) daylight saving or clock changes. It's kind of hinted to in the current description, but reading PEP 418 it's very clear that it's part of the intention, as it falls back to time.monotonic() apart from on `nt`.

Author: anarcat

Doc/library/time.rst

@@ -314,7 +314,8 @@ Functions
    clock with the highest available resolution to measure a short duration.  It
    does include time elapsed during sleep and is system-wide.  The reference
    point of the returned value is undefined, so that only the difference between
-   the results of two calls is valid.
+   the results of two calls is valid. This counter, like :func:`monotonic`, cannot
+   go backwards.
 
    Use :func:`perf_counter_ns` to avoid the precision loss caused by the
    :class:`float` type.