The documentation mentions @Alternative but it does not mention @Specializes which is kind of connected. In certain cases you want to use @Alternative but find out that is not enough and that is when @Specializes comes into play.
Add a subsection to 8.2.8. Alternative Beans where you mention @Specializes.
Weld doc has a great sample showing how you actually get from using alternatives to specialization and why. Something on a similar bases might be added here.
Please note that I am not entirely sure you should add this. It is surely not as common as alternatives and I can see the doc is very basic and general. However it is probably still more common that extensions which are present. So I will leave that up to you whether or not to add this (sub)section.