…toringProvider

Rather than only adapt refactoring actions that conform to
SyntaxRefactoringProvider, which takes a syntax node and produces a
syntax node, adapt to the less-constraining EditRefactoringProvider,
which takes a syntax node and produces edits. We can map edits over
just as effectively.

…unction

This code action takes an undocumented function declaration like

    func refactor(syntax: DeclSyntax, in context: Void) -> DeclSyntax?

and adds stub documentation for the parameters / result / etc., like this:

    /// A description
    /// - Parameters:
    ///   - syntax:
    ///   - context:
    ///
    /// - Returns: