doc: refine the Overriding chapter of the Nixpkgs manual

[ShamrockLee]

Description of changes

This patch refines the Overriding chapter of Nixpkgs manual, which include the general definition of overriding and runnable examples.

It introduces the terms overridables (overridable attribute sets) and overriders (attributes to perform the overriding) to make communication easier.

Cc: @fricklerhandwerk

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 23.11 Release Notes (or backporting 23.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

[fricklerhandwerk]

Triaged in documentation team meeting:

• it doesn't look quite right, but we don't have the resources to review it to conclusion
• very briefly
  • introducing new terms doesn't help and the introduction is probably too abstract
  • it's not exactly clear why we need to make this change
  • having an introduction is good, but this proposal does not really explain where these functions come from

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/2023-11-09-documentation-team-meeting-notes-93/35244/1

[ShamrockLee]

I admit that the motivation to introduce this two terms came from the difficulties encountered when documenting lib.applyToVariable, which sounds like putting the cart before the horse.

We could refer to such an of attribute set as "an overridable attribute set", and such attribute as "the attribute that performs overriding". It's also sensible to prioritize package overriding over the general concept of overriding.

I'll close this PR to preserve the discussion. (It would be confusing for people who follow the discussion thread if the changes are forcefully overwritten.)