Document spacingFactor for preset layout

Ref: Add official support for spacingFactor in the preset layout #3172

src/extensions/layout/preset.js

@@ -8,6 +8,7 @@ let defaults = {
   pan: undefined, // the pan level to set (prob want fit = false if set)
   fit: true, // whether to fit to viewport
   padding: 30, // padding on fit
+  spacingFactor: undefined, // Applies a multiplicative factor (>0) to expand or compress the overall area that the nodes take up
   animate: false, // whether to transition the node positions
   animationDuration: 500, // duration of animation in ms if enabled
   animationEasing: undefined, // easing of animation if enabled