documentation: clarify Overview

[valentinewallace]

w/r/t "This guide aims to give the reader an overview of the structure of Rust Lightning functionality, as well as a reference for integrating the exported bindings in an object-oriented language using Swift as the example."

Some qs:

1. is this guide a walkthrough for how to create language bindings to Rust Lightning's exported C/C++ bindings, using Swift as the example language?
2. and/or, is this guide a walkthrough for how to create a RL node using this repo's Swift language bindings?

I'm still figuring out the answers to these questions, but I think the answers to those questions should be clarified in the opening paragraph. :)