append explanation of element in backdoor (#11849)

* append explanation of element in backdoor

* fix typo

* Update mobile-command.yml

commands-yml/commands/mobile-command.yml

@@ -83,7 +83,7 @@
       | mobile:setTime | Set time by [PickerActions#setTime](https://developer.android.com/reference/android/support/test/espresso/contrib/PickerActions.html#setTime(int,%20int)) for [TimePicker](https://developer.android.com/reference/android/widget/TimePicker). | `{element, hours, minutes}` | `{element: elementId, hours: 12, minutes: 10}` |
       | mobile:navigateTo | Action to [NavigationView](https://developer.android.com/reference/android/support/design/widget/NavigationView) by [NavigationViewActions#navigateTo](https://developer.android.com/reference/android/support/test/espresso/contrib/NavigationViewActions.html#navigateTo(int)). The view must be a child of a DrawerLayout, of type NavigationView, visible on screen and displayed on screen. | `{element, menuItemId}` | `{element: elementId, menuItemId: 1}` |
       | mobile:scrollToPage | Action to [ViewPager](https://developer.android.com/reference/android/support/v4/view/ViewPager) by [ViewPagerActions](https://developer.android.com/reference/android/support/test/espresso/contrib/ViewPagerActions). `scrollTo` must be one of `first`, `last`, `left`, `right`. `scrollTo` is used by default if `scrollTo` and `scrollToPage` are provided. | `{scrollTo, scrollToPage, smoothScroll}` | `{element: elementId, scrollTo: 'left', smoothScroll: true}`, `{element: elementId, scrollToPage: 2}`|
-      | mobile:backdoor | Invoke arbitrary methods defined in Android app. The methods must be `public`. `target` is `activity` or `application`. `methods` are methods you would like to invoke. Read docstring of _mobileBackdoor_ method [here](https://github.com/appium/appium-espresso-driver/blob/master/lib/commands/general.js) for more details | `{target, methods}` | `{target: 'activity', methods: [{name: 'method1'}, {name: 'method2', args: [{value: 'hello appium', type: 'java.lang.String'}] }] }` |
+      | mobile:backdoor | Invoke arbitrary methods defined in Android app. The methods must be `public`. `target` is `activity`, `application` or `element`. `methods` are methods you would like to invoke. `elementId` is mandatory if `target` is `element`. Read docstring of _mobileBackdoor_ method [here](https://github.com/appium/appium-espresso-driver/blob/master/lib/commands/general.js) for more details | `{target, methods, elementId}` | `{target: 'activity', methods: [{name: 'method1'}, {name: 'method2', args: [{value: 'hello appium', type: 'java.lang.String'}] }] }`, `{target: 'element', elementId: elementId, [{name: 'getTypeface'}, {name: 'isItalic'}]}` |
       | mobile:flashElement | Flashes an element on the screen to visually confirm you are operating on the correct element. Can optionally set `durationMillis` (in ms) and `repeatCount` of animation| `{elementId, durationMillis, repeatCount}` | `{elementId: "1234-abcd-5678", durationMillis: 1000, repeatCount: 10}` |
 
   example_usage: