docs(dataarray): update getRanges description to include details on multi-component magnitude

Author: jmannau

Sources/Common/Core/DataArray/index.d.ts

@@ -87,6 +87,10 @@ export interface vtkDataArray extends vtkObject {
    * Returns an array of the ranges for each component of the DataArray.
    * Defaults to computing all the ranges if they aren't already computed.
    *
+   * If the number of components is greater than 1, the last element in the
+   * ranges array is the min,max magnitude of the dataset. This is the same as
+   * calling `getRange(-1)`.
+   *
    * Passing `getRanges(false)` will return  a clone of the ranges that have
    * already been computed. This is useful when you want to avoid recomputing
    * the ranges, which can be expensive.