Document that compiler.extends can be an array

### 🔍 Search Terms

compiler.extends extends

### ✅ Viability Checklist

- [x] This wouldn't be a breaking change in existing TypeScript/JavaScript code
- [x] This wouldn't change the runtime behavior of existing JavaScript code
- [x] This could be implemented without emitting different JS based on the types of the expressions
- [x] This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
- [x] This isn't a request to add a new utility type: https://github.com/microsoft/TypeScript/wiki/No-New-Utility-Types
- [x] This feature would agree with the rest of our Design Goals: https://github.com/Microsoft/TypeScript/wiki/TypeScript-Design-Goals

### ⭐ Suggestion

After merging https://github.com/microsoft/TypeScript/pull/50403, the docs at https://www.typescriptlang.org/tsconfig/#extends haven't been updated to indicate that extends may be an array.

### 📃 Motivating Example

Users may only learn that `extends` may be an array if they find the PR above.

Start at the tsconfig reference page for compilerOptions.extends and review the linked TypeScript PR #50403 for the supported behavior. Update the documentation to state that extends may be an array, then verify the rendered page clearly reflects this without changing other configuration guidance.

Written by the indexing model from the issue text.