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
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Actual behavior:
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Commencez par l’introduction aux declaration files à l’adresse https://www.typescriptlang.org/docs/handbook/declaration-files/introduction.html, puis suivez ses références vers declare global et les recommandations associées concernant les declaration files. Consultez le contenu environnant du handbook avant d’effectuer des modifications. C’est terminé lorsque la documentation définit clairement les declaration files, explique pourquoi et quand utiliser declare, et explique directement le namespace global.