vtkThreshold: Document magnitude option

Documentation/release/dev/add-vtkThreshold-magnitude-option.md

+## Add a magnitude option to vtkThreshold
+
+You can now use input array's magnitude values for thresholding data.
+To do so, you just need to set the selected component (when on SELECTED
+mode) as the number of component of the input array.
+
+```cpp
+// let's say you have a source with a 'Speed' array
+// which has 3 components
+
+vtkNew<vtkThreshold> threshold;
+threshold->SetInputConnection(source->GetOutputPort());
+threshold->SetInputArrayToProcess(0, 0, 0, 0, "Speed");
+
+threshold->SetThresholdFunction(vtkThreshold::THRESHOLD_UPPER);
+threshold->SetUpperThreshold(4.2);
+
+threshold->SetComponentModeToUseSelected();
+threshold->SetSelectedComponent(3); // Select magnitude
+threshold->Update();
+```