Fix build with `documentation.nixos.includeAllModules = true;`

[Ma27]

This includes the options of all modules used in the evaluation, not just the ones from <nixpkgs/nixos> in the local manual.

Right now this breaks with

error: attribute '"‹name›"' missing

because the submodule environment.persistence doesn't have actual declarations when building the manual, but a dummy only to evaluate the sub-options (including their defaults which is the cause of the error) and generate documentation from that.

Using defaultText prevents that because there's no need anymore to evaluate the default values of the options.

cc @lovesegfault @talyz

[lovesegfault]

Thanks!