Android: Expand the comment for NVidiaShieldWorkaroundView

This text has been taken from the message of the commit that added
the class. (I don't have an Nvidia Shield to reproduce the bug with.)

Original commit by JosJuice for Dolphin-emu.

src/android/app/src/main/java/org/citra/citra_emu/ui/NVidiaShieldWorkaroundView.java

@@ -12,6 +12,9 @@ import android.view.View;
 
 /**
  * Work around a bug with the nVidia Shield.
+ *
+ * Without this View, the emulation SurfaceView acts like it has the
+ * highest Z-value, blocking any other View, such as the menu fragments.
  */
 public final class NVidiaShieldWorkaroundView extends View {
     public NVidiaShieldWorkaroundView(Context context, AttributeSet attrs) {