doc: Mark `mad_over_time` as experimental (#13440)

We forgot to do that in
https://github.com/prometheus/prometheus/pull/13059

Signed-off-by: beorn7 <beorn@grafana.com>

docs/querying/functions.md

@@ -644,10 +644,15 @@ over time and return an instant vector with per-series aggregation results:
 * `quantile_over_time(scalar, range-vector)`: the φ-quantile (0 ≤ φ ≤ 1) of the values in the specified interval.
 * `stddev_over_time(range-vector)`: the population standard deviation of the values in the specified interval.
 * `stdvar_over_time(range-vector)`: the population standard variance of the values in the specified interval.
-* `mad_over_time(range-vector)`: the median absolute deviation of all points in the specified interval.
 * `last_over_time(range-vector)`: the most recent point value in the specified interval.
 * `present_over_time(range-vector)`: the value 1 for any series in the specified interval.
 
+If the [feature flag](../feature_flags/)
+`--enable-feature=promql-experimental-functions` is set, the following
+additional functions are available:
+
+* `mad_over_time(range-vector)`: the median absolute deviation of all points in the specified interval.
+
 Note that all values in the specified interval have the same weight in the
 aggregation even if the values are not equally spaced throughout the interval.