lib.fixedPoints.toExtension: improve documentation

    `toExtension` is the `toFunction` for extending functions (a.k.a. extensions or overlays).

    It converts a non-function or a single-argument function to an extending function,

    while returning a double-argument function as-is.

    while returning a two-argument function as-is.

    That is, it takes one of `x`, `prev: x`, or `final: prev: x`,

    and returns `final: prev: x`, where `x` is not a function.

    That is, it takes a value of the shape `x`, `prev: x`, or `final: prev: x`,

    and returns `final: prev: x`, assuming `x` is not a function.

    This function is extracted from the implementation of

    the fixed-point arguments support of `stdenv.mkDerivation`.

    This function takes care of the input to `stdenv.mkDerivation`'s

    `overrideAttrs` function.

    It bridges the gap between `<pkg>.overrideAttrs`

    before and after the overlay-style support,

    as well as `config.packageOverrides` and `config.overlays` in `pkgs`.

    before and after the overlay-style support.

    # Inputs

    `f`

    : The function or non-function to convert to an extending function.

    : The function or value to convert to an extending function.

    # Type

        # f is (prev: { ... })

        fPrev

    else

      # f is { ... }

      # f is not a function; probably { ... }

      final: prev: f;

}