Google recently decided to make my life a lot easier by releasing the changelog for Jelly Bean. Cool!
While, of course, I take issue with its thoroughness, it also wasn't all that accurate when it was first published. It's since been corrected, but the internet never forgets. Check out this Google listing:
Yeah... There isn't actually a ringtone editor in Jelly Bean. Google seems to agree, because, after the initial posting, the mention of the editor was completely removed from the changelog.
So, apparently, the Jelly Bean version of Google Music was supposed to include a ringtone editor, and it was far enough along that it was accidentally included in the changelog.