Update documentation to reflect change in convention

[vpfeiffer]

The convention has changed to use final instead of self and prev instead of super.
This new convention is much easier to understand especially for users that are new to Nix and Nixpkgs.
This change is notable in the Nixpkgs project code.
I added a note mentioning that the official documentation hasn't been update with this information.
The documentation is currently undergoing an overhaul and that is why I believe this hasn't been reflected in
the nixpkgs documentation as of yet.

[vpfeiffer]

I realized I missed that there are some examples in the nixpkgs documentation that use final and prev but just not in the overlays section yet.

[tazjin]

Hey, thanks for the PR!

I've imported it into the TVL Gerrit here (with some minor formatting fixes): https://cl.tvl.fyi/c/depot/+/6859/

(Note that this Github repo is just a split out of TVL's actual monorepo).

As for the change itself, I'm going to ask some of our contributors that are more active nixpkgs contributors than myself to review, as I'm afraid I am not up-to-date with most of the convention changes happening at the moment :)

[tazjin]

Going to close this PR as the review flow is on Gerrit. Note that you can log in there with your Github account and comment, if something comes up!

[vpfeiffer]

No problem! I wanted to add the second comment I mentioned above to the issue in Gerrit but there was an unexpected error when I attempted to sign in with my GH credentials. I'll try again later.

[tazjin]

Oh that's very strange, I've filed an issue on our side for that so someone can look at the logs. Thanks so much for letting me know!

(If that someone ends up being me it'll unfortunately have to wait until tomorrow, I caught a cold 🤒)

[vpfeiffer]

Feel better!