title: What's difference between NSPhotoLibraryAddUsageDescription and NSPhotoLibraryUsageDescription?
body: <p>My app get crashed today while updating on Xcode9, testing on iOS11.
  After adding <code>NSPhotoLibraryAddUsageDescription</code> then it works, even i already had <code>NSPhotoLibraryUsageDescription</code>.</p>
  
  <p>Ive read about them, one supported since iOS6, one iOS11 but Apple didn't mention what's difference between them.
  <a href="https://developer.apple.com/library/content/documentation/General/Reference/InfoPlistKeyReference/Articles/CocoaKeys.html#//apple_ref/doc/uid/TP40009251-SW73" rel="noreferrer">https://developer.apple.com/library/content/documentation/General/Reference/InfoPlistKeyReference/Articles/CocoaKeys.html#//apple_ref/doc/uid/TP40009251-SW73</a></p>
  
  <p>If i keep the new one (<code>NSPhotoLibraryAddUsageDescription</code>), will it work for iOS 8 also or i have to keep both of them?</p>