is out-of-bounds in tuple of length，but the introduction in the document can be out of bounds

TypeScript Version: 3.2.0
Code
This is the official demo

// Declare a tuple type
let x: [string, number];
// Actually，this code will throw error
x[3] = 'world'; // OK, 'string' can be assigned to 'string | number'

Expected behavior:
I want to know if this is a bug or this is document error

Actual behavior:
Tuple out of bounds will trow error, but document say When accessing an element outside the set of known indices, a union type is used instead

Playground Link:
https://www.tslang.cn/play/index.html#src=let%20x%3A%20%5Bstring%2C%20number%5D%3B%0D%0A%2F%2F%20When%20accessing%20an%20element%20outside%20the%20set%20of%20known%20indices%2C%20a%20union%20type%20is%20used%20instead%3A%0D%0A%2F%2F%20Actually%EF%BC%8Cthis%20code%20will%20throw%20error%0D%0Ax%5B3%5D%20%3D%20%22world%22%3B%20%2F%2F%20OK%2C%20'string'%20can%20be%20assigned%20to%20'string%20%7C%20number'

Related Issues: NO

Start by reproducing the tuple access and assignment behavior in the linked TypeScript Playground, then compare it with the official documentation passage quoted in the issue. Update the documentation to accurately explain out-of-bounds tuple access and verify that the example no longer suggests the assignment is valid.