lib.fix: Improve doc more

  /*

    `fix f` computes the fixed point of the given function `f`. In other words, the return value is `x` in `x = f x`.

    `f` is usually returns an attribute set that expects its final, non-recursive representation as an argument.

    `f` must be a lazy function.

    This means that `x` must be a value that can be partially evaluated,

    such as an attribute set, a list, or a function.

    This way, `f` can use one part of `x` to compute another part.

    **How it works**

    **Relation to syntactic recursion**

    For context, Nix lets you define attribute set values in terms of other attributes using the `rec { }` attribute set literal syntax.

    This section explains `fix` by refactoring from syntactic recursion to a call of `fix` instead.

    For context, Nix lets you define attributes in terms of other attributes syntactically using the [`rec { }` syntax](https://nixos.org/manual/nix/stable/language/constructs.html#recursive-sets).

    ```nix

    nix-repl> rec {

    { bar = "bar"; foo = "foo"; foobar = "foobar"; }

    ```

    This is convenient when constructing a value to pass to a function for example, but a similar effect can be achieved with a `let` binding:

    This is convenient when constructing a value to pass to a function for example,

    but an equivalent effect can be achieved with the `let` binding syntax:

    ```nix

    nix-repl> let self = {

    { bar = "bar"; foo = "foo"; foobar = "foobar"; }

    ```

    `let` bindings are nice, but as it is with `let` bindings in general, we may get more reuse out of the code by defining a function.

    But in general you can get more reuse out of `let` bindings by refactoring them to a function.

    ```nix

    nix-repl> f = self: {

    }

    ```

    This is where `fix` comes in. Note that the body of the `fix` function

    looks a lot like our earlier `let` binding, and that's no coincidence.

    Fix is no more than such a recursive `let` binding, but with everything

    except the recursion factored out into a function parameter `f`.

    This is where `fix` comes in, it contains the syntactic that's not in `f` anymore.

    ```nix

    fix = f:

    nix-repl> fix = f:

      let self = f self; in self;

    ```

    So applying `fix` is another way to express our earlier examples.

    By applying `fix` we get the final result.

    ```

    ```nix

    nix-repl> fix f

    { bar = "bar"; foo = "foo"; foobar = "foobar"; }

    ```

    This example did not _need_ `fix`, and arguably it shouldn't be used in such an example.

    However, `fix` is useful when your `f` is a parameter, or when it is constructed from higher order functions.

    Such a refactored `f` using `fix` is not useful by itself.

    See [`extends`](#function-library-lib.fixedPoints.extends) for an example use case.

    There `self` is also often called `final`.

    Type: fix :: (a -> a) -> a

    Example:

      fix (self: { foo = "foo"; bar = "bar"; foobar = self.foo + self.bar; })

      => { bar = "bar"; foo = "foo"; foobar = "foobar"; }

      fix (self: [ 1 2 (elemAt self 0 + elemAt self 1) ])

      => [ 1 2 3 ]

  */

  fix = f: let x = f x; in x;