docs: update function docs on deriv

Signed-off-by: mtfoley <mtfoley.mae@gmail.com>

docs/querying/functions.md

@@ -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.
 
@@ -456,4 +468,4 @@ The following are useful for converting between degrees and radians:
 
 - `deg(v instant-vector)`: converts radians to degrees for all elements in `v`.
 - `pi()`: returns pi.
-- `rad(v instant-vector)`: converts degrees to radians for all elements in `v`.
+- `rad(v instant-vector)`: converts degrees to radians for all elements in `v`.