Document canClickTarget (#465)

Closes #461

docs-src/tutorials/02-usage.md

@@ -174,6 +174,7 @@ the step will execute. For example:
     });
   },
   ```
+- `canClickTarget` A boolean, that when set to false, will set `pointer-events: none` on the target
 - `classes`: A string of extra classes to add to the step's content element.
 - `buttons`: An array of buttons to add to the step. These will be rendered in a footer below the main body text. Each button in the array is an object of the format:
   - `text`: The HTML text of the button

src/js/step.jsx

@@ -78,6 +78,7 @@ export class Step extends Evented {
    * @param {string} options.buttons.button.classes Extra classes to apply to the `<a>`
    * @param {boolean} options.buttons.button.secondary If true, a shepherd-button-secondary class is applied to the button
    * @param {string} options.buttons.button.text The HTML text of the button
+   * @param {boolean} options.canClickTarget A boolean, that when set to false, will set `pointer-events: none` on the target
    * @param {string} options.classes A string of extra classes to add to the step's content element.
    * @param {string} options.highlightClass An extra class to apply to the `attachTo` element when it is
    * highlighted (that is, when its step is active). You can then target that selector in your CSS.