Added example as FAQ

[franciscop]

Based on this issue, added an example of how to check the return type:

(added content follows)

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Checking the type of the return value

To check the return value of a library, simply set the type to the variable in your index.d.ts:

const test: string = await myFunction();

If the function only returns a string a never a number, then this will fail:

// FAILS
const test: number = await myFunction();

[ai]

We use different control comment for errors. And we need to add error text to it (to be sure that it is exactly the conflict with return type