Clear up several issues around WebAssembly.Function usage

[tsyesika]

This clears up two issues in the overview document surrounding the usage of WebAssembly.Function, these are:

• What behaviour should occur if you pass a WebAssembly.Function object to the WebAssembly.Function constructor. (discussed here: #13)
• Clarify that the same behaviour should occur when calling the function as when calling an exported function (e.g. the ToJSValue should be applied to arguments when called, etc.)

[wingo]

cc @Ms2ger