Merge pull request #431 from brian-brazil/hw

Document holt_winters()

content/docs/querying/functions.md

@@ -158,6 +158,16 @@ for quantiles located in the lowest bucket.
 If `b` contains fewer than two buckets, `NaN` is returned. For φ < 0, `-Inf` is
 returned. For φ > 1, `+Inf` is returned.
 
+## `holt_winters()`
+
+`holt_winters(v range-vector, sf scalar, tf scalar)` produces a smoothed value
+for time series based on the range in `v`. The lower the smoothing factor `sf`,
+the more importance is given to old data. The higher the trend factor `tf`, the
+more trends in the data is considered. Both `sf` and `tf` must be between 0 and
+1.
+
+`holt_winters` should only be used with gauges.
+
 ## `increase()`
 
 `increase(v range-vector)` calculates the increase in the