Clarify that `deriv` is per-second.

content/docs/querying/functions.md

@@ -87,7 +87,7 @@ delta(cpu_temp_celsius{host="zeus"}[2h])
 
 ## `deriv()`
 
-`deriv(v range-vector)` calculates the derivative of the time series in a range
+`deriv(v range-vector)` calculates the per-second derivative of the time series in a range
 vector `v`, using [simple linear regression](http://en.wikipedia.org/wiki/Simple_linear_regression).
 
 `deriv` should only be used with gauges.