t.tsUnknownType is documented but doesn't exist

## Bug Report

**Current Behavior**
Looking at the documentation for type there is a [tsUnknownType](https://babeljs.io/docs/en/babel-types.html#tsunknowntype), but calling the function gives a `TypeError: t.tsUnknownType is not a function`

**Input Code**
```js
const generate = require("@babel/generator").default;
const t = require("@babel/types");

const ast = t.tsUnknownType([t.tsAnyKeyword()]);

const { code } = generate(ast);
console.log(code);
```

**Expected behavior/code**
I believe the output should be `unknown`.

**Babel Configuration (.babelrc, package.json, cli command)**
N/A, this is more a tooling bug than babel behavior bug

**Environment**
- Babel version(s): 7.5.5
- Node/npm version: Node v10.12.0 / yarn 1.17.3
- OS: Ubuntu 18.04
- How you are using Babel: `api`

**Possible Solution**
Searching through this repo yields nothing, it might not exist (yet)?