Issue 10231 - Document typed alias parameters.

https://issues.dlang.org/show_bug.cgi?id=10231

Note that I haven't touched the `Specialization` section yet because I've just found a new specialization bug with typed alias parameters - [Issue 12620](https://issues.dlang.org/show_bug.cgi?id=12620). Since this is a relatively new feature it's probably best to keep the non-working parts out of the spec for now.