docs: update function docs on deriv
@@ -130,6 +130,11 @@ delta(cpu_temp_celsius{host="zeus"}[2h])
`deriv(v range-vector)` calculates the per-second derivative of the time series in a range
vector `v`, using [simple linear regression](https://en.wikipedia.org/wiki/Simple_linear_regression).
+The range vector must have at least two samples in order to perform the calculation. The linear
+regression is used to calculate a slope and offset for a trend line, and the slope value is used
+for the derivative calculation. In order to reduce calculation errors, the timestamp of the first
+sample in the range vector is set as `x=0` within the linear regression algorithm. When `+Inf` or
+`-Inf` are found in the range vector, the slope and offset value calculated will be `NaN`.
`deriv` should only be used with gauges.
@@ -329,6 +334,13 @@ January etc.
`predict_linear(v range-vector, t scalar)` predicts the value of time series
`t` seconds from now, based on the range vector `v`, using [simple linear
regression](https://en.wikipedia.org/wiki/Simple_linear_regression).
+The range vector must have at least two samples in order to perform the
+calculation. The linear regression is used to calculate a slope and offset
+for a trend line, and the slope and offset are used to calculate the predict
+the value of the time series. In order to reduce calculation errors, the
+timestamp of the first sample in the range vector is set as `x=0` within the
+linear regression algorithm. When `+Inf` or `-Inf` are found in the range vector,
+the slope and offset value calculated will be `NaN`.
`predict_linear` should only be used with gauges.
