nixos/README.md: relax the requirement of providing option defaults

[pbsds]

If we enforce the requirement of option defaults to the extreme, then we end up with silly sentinel values, a bunch of nullable types, and assertions checking for sentinel values.

Description of changes

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/)
• 24.11 Release Notes (or backporting 23.11 and 24.05 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.

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Add a 👍 reaction to pull requests you find important.

[pbsds]

motivating example

• nixos/bluemap: fix defaults issue with services.bluemap.host #320836

[Aleksanaa]

Suggested change

	- Defaults may only be omitted if: (1) the user configuring them is required and (2) they are not used/block evaluation when the the module is not enabled.
	- Defaults may only be omitted if:
	1. the user configuring them is required
	2. they are not used/block evaluation when the the module is not enabled

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