Update documentation for PermissionsAndroid

Summary:
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Current documentation: [https://facebook.github.io/react-native/docs/permissionsandroid.html](https://facebook.github.io/react-native/docs/permissionsandroid.html)

PermissionsAndroid must be imported for the example to work, otherwise the error "Reference Error: can't find variable PermissionsAndroid" will be shown.
https://stackoverflow.com/questions/42434124/why-permissionandroid-doesnt-work

None - Just documentation changes.
Closes #15505

Differential Revision: D5643004

Pulled By: TheSavior

fbshipit-source-id: 5ed81fd0120270da9ab180dd3f1cb569544e0520

Libraries/PermissionsAndroid/PermissionsAndroid.js

@@ -48,6 +48,8 @@ type PermissionStatus = 'granted' | 'denied' | 'never_ask_again';
  *
  * ### Example
  * ```
+ * import { PermissionsAndroid } from 'react-native';
+ *
  * async function requestCameraPermission() {
  *   try {
  *     const granted = await PermissionsAndroid.request(