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/computing/slurm/slurm.nix

@@ -281,6 +281,10 @@ in
         type = types.path;
         internal = true;
         default = etcSlurm;
+        defaultText = literalDocBook ''
+          Directory created from generated config files and
+          <literal>config.${opt.extraConfigPaths}</literal>.
+        '';
         description = ''
           Path to directory with slurm config files. This option is set by default from the
           Slurm module and is meant to make the Slurm config file available to other modules.