Mapped types mistaken for index signature type

Hi @basarat!

I believe the chapter on index signature types may contain some slightly misleading information, although I could be wrong.

In the section title [Using a limited set of string literals](https://github.com/basarat/typescript-book/blob/master/docs/types/index-signatures.md#using-a-limited-set-of-string-literals), the example uses mapped types, which are a separate feature from index signature types.

As far as I understand, index signatures define types for all key lookups, whereas mapped types are a tool for programmatically creating interfaces with only known keys.

It took me awhile to understand the difference myself, especially as the syntax are so similar for the two:

``` ts
// Index signature
{ [index: string]: string }
// Mapped type
{ [key in 'foo']: string }
// … generates:
{ foo: string }
// The generated type has no index signature
```

Index signatures are useful for creating dictionaries aka maps, and mapped types are useful for creating records. https://gist.github.com/OliverJAsh/2a538639e35db183b0cc16ca8ab520a7

What do you think? My only concern is that mapped types are referred to as index signatures in this chapter, but as you can see from the example above, a mapped type does not generate a type with an index signature. They are different tools. 🤔

Start with docs/types/index-signatures.md and review the “Using a limited set of string literals” section alongside the issue’s TypeScript examples. Clarify whether the example uses a mapped type rather than an index signature, and consider explaining the distinction; the documentation is done when the terminology and example accurately distinguish known keys from all key lookups.

Written by the indexing model from the issue text.