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).

home/git.nix

@@ -1,6 +1,5 @@
-# Version control: git + delta pager + commitizen + lazygit. The committer
-# identity (name, email, signing key) comes from the per-user `identity` arg,
-# derived from the registry (users/registry.nix) by mkHost.
+# Version control: git + delta + commitizen + lazygit. Committer identity comes
+# from the per-user `identity` arg (the registry). See README "Users".
 {
   pkgs,
   lib,
@@ -20,8 +19,7 @@ in
     package = pkgs.gitFull;
     settings = {
       user.name = identity.fullName;
-      # Identity from the registry. mkDefault so a host-specific module can still
-      # override it without conflicting.
+      # mkDefault so a host-specific module can still override it.
       user.email = lib.mkDefault identity.email;
       push.autoSetupRemote = true;
       init.defaultBranch = "main";
@@ -78,9 +76,8 @@ in
         cc = "!cz commit";
       };
 
-      # SSH commit signing, key from the registry. mkDefault on the key and on
-      # gpgsign so a host without that key in its ssh-agent can override gpgsign
-      # to false rather than fail every commit.
+      # SSH signing, key from the registry. mkDefault so a host lacking the key
+      # in its agent can set gpgsign = false instead of failing every commit.
       gpg.format = "ssh";
       user.signingkey = lib.mkDefault identity.signingKey;
       commit.gpgsign = lib.mkDefault true;