Improved sync class constructors

[benStre]

Adds better typing for constructors in sync classes.

Before:

@sync class MyClass {
   // for typing 
   constructor(a: number, b: string){}
   // actual constructor
   @constructor construct(a: number, b: string){
      // ...
    }
}

Now:

@sync class MyClass {
   // actual constructor
   @constructor construct(a: number, b: string){
      // ...
    }
}

(The constructor must be named construct)

Also updated the docs for sync classes