docs: simplify code comments, keep the detail in the README

The registry / multi-user / portable-home rationale lives in the README
(Users, Hosts, Portable home); the corresponding code comments are reduced
to one-liners that point there. No functional change (host derivations are
unchanged).

modules/ssh.nix

@@ -1,9 +1,6 @@
-# Key-only SSH hardening, imported by the hosts that run sshd (T400, Mac Pro,
-# RPi5). The host config still does `services.openssh.enable = true` and opens
-# port 22 next to where it documents the listening service; this module only
-# tightens the policy so a host opting into sshd cannot accidentally ship
-# password/root login. Authorized keys are owned per-user by the identity
-# registry (users/registry.nix, applied via modules/users.nix), not here.
+# Key-only sshd hardening, imported by hosts that run sshd (T400, Mac Pro,
+# RPi5). Authorized keys are owned per-user by the registry (modules/users.nix),
+# not here.
 { ... }:
 {
   services.openssh.settings = {