bpo-29677: DOC: clarify documentation for round (python#357)

* DOC: clarify documentation for `round`

Clarified that `round` can take a negative value for *ndigits*.

* DOC: remove trailing whitespace in previous commit

remove trailing whitespace in previous commit

(cherry picked from commit 6003db7)

Author: gerritholl

Doc/library/functions.rst

@@ -1241,7 +1241,8 @@ are always available.  They are listed here in alphabetical order.
    closest multiple of 10 to the power minus *ndigits*; if two multiples are
    equally close, rounding is done toward the even choice (so, for example,
    both ``round(0.5)`` and ``round(-0.5)`` are ``0``, and ``round(1.5)`` is
-   ``2``).  The return value is an integer if called with one argument,
+   ``2``).  Any integer value is valid for *ndigits* (positive, zero, or
+   negative).  The return value is an integer if called with one argument,
    otherwise of the same type as *number*.
 
    .. note::