The few that exist might be documented (but please enlighten us on where), but if they are, it's most likely because they are so rare.
"I personally invested in a fund 12 years ago which has returned a compound 21% pa return over that period, so I know that they definitely exist."
That's exactly what the GP says - of course there are, but how do you which ones will be, and know you didn't stumble into this one by chance? Your story might just be survivor bias. When I look at the funds my banks offer, there are some that offer 10%+ returns for the last years. Thing is though that they offer dozens of funds, all of which look equally likely in their prospects. How do I know which one to pick? Their yields are normally distributed, just as one would expect.