Xcode 8.2.1 not showing documentation description on autocomplete

You are right, the descriptions you added to the top of your methods and properties don't appear in the popover anymore.

As noted, you can only see the descriptions of Apple's own methods and properties.
The reason being that Xcode doesn't parse these from their classes but rather from a separate documentation set (which you can find in Xcode's Help/Documentation and API reference tab).

So unless Apple decides to change this, I'm afraid it won't be possible to see your own in the popover.

You could keep an eye on existing doc set generators (AppleDoc, Jazzy), maybe they'll offer a way to link their documentation to Xcode's popover.

Keep in mind that you do see your own comments when opening the quick help popover with alt + click on a method or property.

For me the best way to resolve this is by cleaning the project Shift+Command+K, and if that is not working, it is a god idea to remove Derived Data folder.

To remove this folder go to Xcode preferences, Locations tab

and click on the small arrow to open a finder, and remove manually the folder.

Restart Xcode, and check if now is working

https://developer.apple.com/library/content/documentation/Xcode/Reference/xcode_markup_formatting_ref/MarkupFunctionality.html#//apple_ref/doc/uid/TP40016497-CH54-SW1

please refer the official documentation