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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Start with the declaration-files introduction at https://www.typescriptlang.org/docs/handbook/declaration-files/introduction.html, then trace its references to declare global and related declaration-file guidance. Review the surrounding handbook material before editing. Done means the documentation clearly defines declaration files, explains why and when to use declare, and directly explains the global namespace.