docs: correct stale claims found while reviewing the skills

No host defines a *-nixos-tests output; the pod firewall drops 5900-5903
from pod sources; the mihomo core keeps geodata and cache in its
StateDirectory; WA-020 was used twice and the cited WA-028 had no row.

docs/nixos-modernization-options.md

@@ -1,8 +1,8 @@
 # NixOS Modernization — Options and Decision Record
 
 > **Purpose:** let the maintainer decide, per candidate, whether it is worth adopting — by comparing
-> it against what this repository does **today**. The plan documents under `docs/plans/` are
-> downstream of this file: a plan item is only approved work once its card here says **adopt**.
+> it against what this repository does **today**. A plan derived from this file is only approved
+> work once its card here says **adopt**.
 >
 > Researched against nixpkgs `nixos-unstable` on 2026-10-07. Option names and upstream wording are
 > quoted from the module sources cited in each card.