docs: improve docs for 'lib.attrsets.recursiveUpdateUntil'

[hsjobeki]

Improve the docs for recursiveUpdateUntil

• Inconsistent grammar usage merge vs update
• Inconsistent use of left attribute set vs first attribute set.
• Break up the long single sentence into individual more clear focused sentences.
• fix variable namings

Things done

• Built on platform:
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• Tested, as applicable:
  • NixOS tests in nixos/tests.
  • Package tests at passthru.tests.
  • Tests in lib/tests or pkgs/test for functions and "core" functionality.
• Ran nixpkgs-review on this PR. See nixpkgs-review usage.
• Tested basic functionality of all binary files, usually in ./result/bin/.
• Nixpkgs Release Notes
  • Package update: when the change is major or breaking.
• NixOS Release Notes
  • Module addition: when adding a new NixOS module.
  • Module update: when the change is significant.
• Fits CONTRIBUTING.md, pkgs/README.md, maintainers/README.md and other READMEs.

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[roberth]

We can be more explicit, and there's more to say :)

[hsjobeki]

@roberth i've applied most of your changes. But i have omitted some of the formal definitions. I don't think they are really necessary. This should be a nice improvement now. If we notice that we need more formal definitions we can add them later if that sounds good to you.

[mdaniels5757]

Rebased and ran treefmt.