doc improvement: define a new type Loader<NixExpr>

[bryango]

Hi! I was trying to use a matcher and was confused by the

... ({ self, super, root, ... } -> Path -> a ) ...

everywhere in the doc. Only after I read the source did I realize that this is simply a loader. I believe it would be easier to define:

Loader<a> := { self, super, root, ... } -> Path -> a

... at the top of e.g. loaders and use that instead. Do you think this is a good idea? I am not sure if Loader<a> := is the nix way to annotate a new type, though; the angled brackets are simply borrowed from rust. Also, replacing some of the a with e.g. NixExpr would make it feel more descriptive to me.

If you all agree with this proposal but don't have time to implement this, I can try to submit a pull request, some time (I am very slow though, haha).