Doc pdist's keyword argument w

Missed documenting `w` when adding support for the `wminkowski` metric
in `pdist`. So this adds documentation for that parameter and notes that
it is mandatory (no default).

dask_distance/__init__.py

@@ -138,6 +138,7 @@ def pdist(X, metric="euclidean", **kwargs):
                     (default: estimated from X)
         VI:         Inverse of the covariance matrix for mahalanobis only
                     (default: estimated from X)
+        w:          1-D array of weights for wminkowski only (required)
 
     Returns:
         array:      condensed distance between each pair