lib/options: Relax showOption quoting

https://github.com/NixOS/nixpkgs/commit/124cccbe3b63122733e02e41e45a383ec48752fd
broke the build of NixOS manual.

It does not make sense to be as strict as with attributes since we
are not limited by the CLI's inability to handle numbers.
Placeholders should not be quoted either as they are not part of Nix
syntax but a meta-level construct.