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).

users/registry.nix

@@ -1,14 +1,5 @@
-# User identity registry -- pure data, no machinery. Keyed by username.
-#
-# Each entry describes WHO a user is (display name, email, authorized/signing
-# keys, supplementary groups). The reusable modules read this indirectly:
-#   - system: modules/users.nix builds users.users.* from `userRegistry`
-#     (specialArg) restricted to the host's `hostUsers` set.
-#   - home:   home/git.nix and home/desktop.nix read the per-user `identity`
-#     module arg, which mkHost derives from the matching registry entry.
-#
-# The login name is injected by mkHost as `identity.username`, so it is not
-# repeated inside each entry here.
+# User identity registry -- pure data, keyed by username. See README "Users".
+# (`identity.username` is injected by mkHost, so it is not repeated here.)
 {
   lyrathorpe = {
     fullName = "Lyra Thorpe";
@@ -30,10 +21,7 @@
       "wheel"
       "docker"
     ];
-    # No personal authorized key on file yet. The previous shared user module
-    # applied Lyra's key to every account, including this one -- a defect. Leave
-    # this empty until a real key is provisioned; SSH login is moot on the WSL
-    # host (entered via wsl.exe, not sshd).
+    # No personal key on file yet; add one if SSH login as emmathorpe is wanted.
     sshAuthorizedKeys = [ ];
     signingKey = "key::ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIAJMVgeRKnfX1G8coU3nAobI485aeUpGTMqH7+zbKI8o emma.thorpe@cloud.com";
   };