Improve documentation for stateVersion

The meaning of stateVersion often leads to a lot of confusion. This
commit trys to improve that situation.

nixos/modules/installer/tools/tools.nix

@@ -159,10 +159,12 @@ in
         #   extraGroups = [ "wheel" ]; # Enable ‘sudo’ for the user.
         # };
 
-        # This value determines the NixOS release with which your system is to be
-        # compatible, in order to avoid breaking some software such as database
-        # servers. You should change this only after NixOS release notes say you
-        # should.
+        # This value determines the NixOS release from which the default
+        # settings for stateful data, like file locations and database versions
+        # on your system were taken. It‘s perfectly fine and recommended to leave
+        # this value at the release version of the first install of this system.
+        # Before changing this value read the documentation for this option
+        # (e.g. man configuration.nix or on https://nixos.org/nixos/options.html).
         system.stateVersion = "${config.system.nixos.release}"; # Did you read the comment?
 
       }

nixos/modules/misc/version.nix

@@ -61,11 +61,18 @@ in
         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 can set the
+        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 option
+        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.
       '';
     };