UDA are not covered in documentation and tour

[Fra Mecca]

For the past few days I have been eager to understand more about 
UDA and the only useful links I found are an old discussion about 
implementation by Walter and this:
http://ddili.org/ders/d.en/uda.html
by Ali Çehreli that definitely does the work of explaining it all.

I think we should add a section to the documentation. Is it 
overkill to include them in the Tour?

Or am I wrong and UDA are already covered somewhere in this site?