treewide: Fix types of stripDebugList attrs (and fix doc)

[gnprice]

Motivation for this change

The documentation says this should be a list, and it already is in about half the expressions that set it.

The difference doesn't matter at present, because the elements of these are all space-free literals. But it will in a future with __structuredAttrs .

(The similar attr stripAllList has no users in the nixpkgs tree, so there's nothing to do to fix any of those up.)

Also fix the default value mentioned in the doc, as I noticed when grepping that it was out of date.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
  • (This finds nothing to rebuild, as intended.)
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.