Add read-only options

These are options that can have only one definition, regardless of
priority.

nixos/doc/manual/options-to-docbook.xsl

@@ -39,6 +39,10 @@
                   <emphasis>Type:</emphasis>
                   <xsl:text> </xsl:text>
                   <xsl:apply-templates select="attr[@name = 'type']" mode="top" />
+                  <xsl:if test="attr[@name = 'readOnly']/bool/@value = 'true'">
+                    <xsl:text> </xsl:text>
+                    <emphasis>(read only)</emphasis>
+                  </xsl:if>
                 </para>
               </xsl:if>
 

nixos/modules/misc/version.nix

@@ -29,7 +29,7 @@ with lib;
     };
 
     system.nixosRelease = mkOption {
-      internal = true;
+      readOnly = true;
       type = types.str;
       default = readFile "${toString pkgs.path}/.version";
       description = "NixOS release.";
@@ -48,7 +48,7 @@ with lib;
     };
 
     system.nixosCodeName = mkOption {
-      internal = true;
+      readOnly = true;
       type = types.str;
       description = "NixOS release code name.";
     };