Issue 11705 - Document Typecons and its parameters properly.

https://issues.dlang.org/show_bug.cgi?id=11705

I've removed the second overload because it's unnecessary, and also because the compiler emits ddoc warnings about missing parameters when I try to reference the params from the first overload (we could consider filing an enhancement). Here's what I mean:

```D
/**
Foo

Params:

T2 = An optional parameter.

*/
alias Foo(T) = .Foo!(T, int);

/// ditto
alias Foo(T1, T2) = T1;
```

```
dmd -o- -D -w test.d
test.d(9): Warning: Ddoc: function declaration has no parameter 'T2'
```