Docs: document what directory the parameters to .addFiles...() method family should be relative to

As per title, please document this at https://ts-morph.com/setup/adding-source-files and remove a 100% certain stumbler necessitating source code inspection or trial and error for every new user.

Open https://ts-morph.com/setup/adding-source-files and review the existing documentation for the .addFiles...() method family. Determine which directory the parameters are relative to, then document that relationship on the page; done means a new user no longer needs source inspection or trial and error to understand the paths.

Written by the indexing model from the issue text.