Clarify class property initializer behavior

## The problem Briefly, if a class property isn't initialized with the declaration, it will be initialized as `undefined`, no matter how you initialize it in constructor. This will lead to v8 performance issue because of property type transition in many cases. So I think it's very important to mention the behavior difference in documentation. ## Documentation difference In [Class field declarations](https://github.com/tc39/proposal-class-fields#fields-without-initializers-are-set-to-undefined) proposal from TC39, it's very clearly mentioned that "Fields without initializers are set to `undefined`." Thus in TS handbook, there is nowhere mentioned how the field declaration without initializer will be handled. Actually the documentation lead us to believe it's the same result to initialize a property in declaration or in constructor. E.G., in the [Classes default example](https://www.typescriptlang.org/docs/handbook/classes.html#classes), it says: > In the last line we construct an instance of the Greeter class using new. This calls into the constructor we defined earlier, creating a new object with the Greeter shape, and running the constructor to initialize it. But actually it's like the TC39 proposal, **if you don't assign a default value in declaration, the property will be initialized as `undefined` in constructor (before your actual initialize code).** Take a look at a simple example ``` class Test { x: number; constructor(x: number) { this.x = x; } } ``` With tsc, this will be generated as the following js ``` "use strict"; var Test = /** @class */ (function () { function Test(x) { Object.defineProperty(this, "x", { enumerable: true, configurable: true, writable: true, value: void 0 }); this.x = x; } return Test; }()); ``` As you can see, property `this.x` is firstly defined with `void 0` (undefined) and then assigned with `x` parameter. ## Why should we care …