Inform about useDefineForClassFields breaking change in TS >=4.3

[rangoo94]

While I've been upgrading the TypeScript, I encountered multiple strange issues, related to abstract classes. It took me a while to detect what was the problems reason, as I haven't found in any notes information about useDefineForClassFields option.

It's changing the default behaviour, which may easily break the existing code. I added simplest example as possible in this PR, as I felt that my specific example may be too specific, while this simplified example may help to identify issue anyway.

The exact problem I encountered:

abstract class Parent {
    public constructor() {
        this.run();
    }

    protected abstract run(): void;
}

class Child extends Parent {
    public value!: string;

    protected run() {
        this.value = "test value";
    }
}

console.log(new Child().value);
// for `useDefineForClassFields: false` -> "test value"
// for `useDefineForClassFields: true`  -> undefined

Took me a while to detect the culprit, so I think that it's worth to add information about that to documentation, so it will reduce problems with migration.

[ghost]

All CLA requirements met.