docs(terra-draw): add section on updateModeOptions to MODES guide (#491)

Author: JamesLMilner

guides/4.MODES.md

@@ -169,6 +169,21 @@ We can also provide a `toCustom` function which allows snapping to some arbitrar
   })
 ```
 
+#### Updating a Modes Options After Instantiation
+
+If we want to dynamically change an option passed to a mode after it has been instantiated we can do this using the `updateModeOptions` method. It is used in the following way:
+
+```typescript
+draw.updateModeOptions<typeof TerraDrawPolygonMode>('polygon', {
+  snapping: {
+    toLine: false,
+    toCoordinate: true
+  }
+})
+```
+
+Here we can pass a partial object with the options we want to update. In the case above we are just updating the snapping option. This functionality can be used to change how your modes work on the fly.
+
 #### Projections in Drawing Modes
 
 As we move forward Terra Draw will work on supporting Web Mercator maps out the box with the ability to support Globes (i.e., 3D spherical representations of the earth with no projection) as a secondary option. This is made slightly more complicated by the fact we know sometimes users want to draw geodesic geometries on a web mercator map, for example a geodesic circle or a great circle line. In future we will better align by assuming developers want web mercator first behaviours, with secondary support for globes via the `projection` property for built in modes.