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

<!-- 🚨 STOP 🚨 𝗦𝗧𝗢𝗣 🚨 𝑺𝑻𝑶𝑷 🚨 I think this should be the document not updated，Please update the official documentation，Because it makes people confused --> <!-- Please try to reproduce the issue with `typescript@next`. It may have already been fixed. --> **TypeScript Version:** 3.2.0 **Code** This is the official demo ```ts // 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:** <!-- A link to a TypeScript Playground "Share" link which demonstrates this behavior --> 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:** <!-- Did you find other bugs that looked similar? --> NO