Issue 27936: Update doc for round() to indicate that None is an allowable argument.

Doc/library/functions.rst

@@ -1234,8 +1234,8 @@ are always available.  They are listed here in alphabetical order.
 .. function:: round(number[, ndigits])
 
    Return the floating point value *number* rounded to *ndigits* digits after
-   the decimal point.  If *ndigits* is omitted, it returns the nearest integer
-   to its input.  Delegates to ``number.__round__(ndigits)``.
+   the decimal point.  If *ndigits* is omitted or is ``None``, it returns the
+   nearest integer to its input.  Delegates to ``number.__round__(ndigits)``.
 
    For the built-in types supporting :func:`round`, values are rounded to the
    closest multiple of 10 to the power minus *ndigits*; if two multiples are