bpo-37624: Document weight assumptions for random.choices() (GH-14855)

Doc/library/random.rst

@@ -160,7 +160,8 @@ Functions for sequences
 
    The *weights* or *cum_weights* can use any numeric type that interoperates
    with the :class:`float` values returned by :func:`random` (that includes
-   integers, floats, and fractions but excludes decimals).
+   integers, floats, and fractions but excludes decimals).  Weights are
+   assumed to be non-negative.
 
    For a given seed, the :func:`choices` function with equal weighting
    typically produces a different sequence than repeated calls to