Add documentation to methods

[salvo-polizzi]

I think that could be useful to add some documentation in all methods as a comment.
For example something like that:

def add_book(context: CallbackContext, chat_id: int, isbn: str, title: str, authors: str) -> None:   
"""called in the request of a book or in selling books.
     Add a book in the Books table

        Args:
            context: context passed by the handler
            chat_id: id of the chat of the message
            isbn: isbn of the book to add
            title: title of the book to add
            authors: authors of the book to add"""