treewide: add literalDocBook text to options with complex defaults

some options have default that are best described in prose, such as
defaults that depend on the system stateVersion, defaults that are
derivations specific to the surrounding context, or those where the
expression is much longer and harder to understand than a simple text
snippet.

nixos/modules/services/web-apps/keycloak.nix

@@ -140,6 +140,7 @@ in
           lib.mkOption {
             type = lib.types.port;
             default = dbPorts.${cfg.database.type};
+            defaultText = lib.literalDocBook "default port of selected database";
             description = ''
               Port of the database to connect to.
             '';

nixos/modules/services/web-apps/nextcloud.nix

@@ -499,6 +499,7 @@ in {
     occ = mkOption {
       type = types.package;
       default = occ;
+      defaultText = literalDocBook "generated script";
       internal = true;
       description = ''
         The nextcloud-occ program preconfigured to target this Nextcloud instance.