Add comment that empty constructors are needed for IcePick

See 5e7ad6ffd1 and https://github.com/TeamNewPipe/NewPipe/pull/10781#discussion_r1545351144

app/src/main/java/org/schabi/newpipe/fragments/detail/DescriptionFragment.java

@@ -31,6 +31,7 @@ public class DescriptionFragment extends BaseDescriptionFragment {
     }
 
     public DescriptionFragment() {
+        // keep empty constructor for IcePick when resuming fragment from memory
     }
 
 

app/src/main/java/org/schabi/newpipe/fragments/list/channel/ChannelAboutFragment.java

@@ -31,6 +31,7 @@ public class ChannelAboutFragment extends BaseDescriptionFragment {
     }
 
     public ChannelAboutFragment() {
+        // keep empty constructor for IcePick when resuming fragment from memory
     }
 
     @Override