nixos/version: rewrite stateVersion documentation

Match the comment in nixos-generate-config and add some more details for the curious.

nixos/modules/misc/version.nix

@@ -121,22 +121,32 @@ in
       default = cfg.release;
       defaultText = literalExpression "config.${opt.release}";
       description = lib.mdDoc ''
-        Every once in a while, a new NixOS release may change
-        configuration defaults in a way incompatible with stateful
-        data. For instance, if the default version of PostgreSQL
-        changes, the new version will probably be unable to read your
-        existing databases. To prevent such breakage, you should set the
-        value of this option to the NixOS release with which you want
-        to be compatible. The effect is that NixOS will use
-        defaults corresponding to the specified release (such as using
-        an older version of PostgreSQL).
-        It’s perfectly fine and recommended to leave this value at the
-        release version of the first install of this system.
-        Changing this option will not upgrade your system. In fact it
-        is meant to stay constant exactly when you upgrade your system.
-        You should only bump this option, if you are sure that you can
-        or have migrated all state on your system which is affected
-        by this option.
+        This option defines the first version of NixOS you have installed on this particular machine,
+        and is used to maintain compatibility with application data (e.g. databases) created on older NixOS versions.
+
+        For example, if NixOS version XX.YY ships with AwesomeDB version N by default, and is then
+        upgraded to version XX.YY+1, which ships AwesomeDB version N+1, the existing databases
+        may no longer be compatible, causing applications to fail, or even leading to data loss.
+
+        The `stateVersion` mechanism avoids this situation by making the default version of such packages
+        conditional on the first version of NixOS you've installed (encoded in `stateVersion`), instead of
+        simply always using the latest one.
+
+        Note that this generally only affects applications that can't upgrade their data automatically -
+        applications and services supporting automatic migrations will remain on latest versions when
+        you upgrade.
+
+        Most users should **never** change this value after the initial install, for any reason,
+        even if you've upgraded your system to a new NixOS release.
+
+        This value does **not** affect the Nixpkgs version your packages and OS are pulled from,
+        so changing it will **not** upgrade your system.
+
+        This value being lower than the current NixOS release does **not** mean your system is
+        out of date, out of support, or vulnerable.
+
+        Do **not** change this value unless you have manually inspected all the changes it would
+        make to your configuration, and migrated your data accordingly.
       '';
     };