Documentation: Clarify declaration files and the declare keyword

**TypeScript Version:** 2.8.0 **Search Terms:** document declaration files ** Issue** It doesn't look like you actually define what a declaration file is in your documentation. For instance, your introduction to these at https://www.typescriptlang.org/docs/handbook/declaration-files/introduction.html tells me you're going to help me learn how to write them, but not what they are or why I'd want one. But, I don't think this makes it easy for a newbie to understand. Similarly, you don't seem to explain when the `declare` keyword is, or when it should be used (as far as I can tell, it is used when you need to tell the compiler that the module/library/whatever has a value of some type). You also seem to make some references to the global namespace (`declare global { ... }`)), but don't seem to directly explain it anywhere. **Code** n/a **Expected behavior:** n/a **Actual behavior:** n/a **Playground Link:** n/a **Related Issues:** n/a