lib.modules.doRename: Add doc comments

I don't think these are rendered yet, but this at least provides
the content.

Follow-up to #285612

lib/modules.nix

@@ -1256,7 +1256,68 @@ let
       (opt.highestPrio or defaultOverridePriority)
       (f opt.value);
 
-  doRename = { from, to, visible, warn, use, withPriority ? true, condition ? true }:
+  /*
+    Return a module that help declares an option that has been renamed.
+    When a value is defined for the old option, it is forwarded to the `to` option.
+   */
+  doRename = {
+    # List of strings representing the attribute path of the old option.
+    from,
+    # List of strings representing the attribute path of the new option.
+    to,
+    # Boolean, whether the old option is to be included in documentation.
+    visible,
+    # Whether to warn when a value is defined for the old option.
+    # NOTE: This requires the NixOS assertions module to be imported, so
+    #        - this generally does not work in submodules
+    #        - this may or may not work outside NixOS
+    warn,
+    # A function that is applied to the merged option value, to form the value
+    # of the old `from` option. Example: ``lib.warn "Obsolete option `${opt.old}' is used. Use `${opt.to}' instead."``
+    use,
+    # Legacy option, enabled by default: whether to preserve the priority of definitions in `old`.
+    withPriority ? true,
+    # A boolean that defines the `mkIf` condition for `to`.
+    # If the condition evaluates to `true`, and the `to` path points into an
+    # `attrsOf (submodule ...)`, then `doRename` would cause an empty module to
+    # be created, even if the `from` option is undefined.
+    # By setting this to an expression that may return `false`, you can inhibit
+    # this undesired behavior.
+    #
+    # Example:
+    #
+    # ```nix
+    # { config, lib, ... }:
+    # let
+    #   inherit (lib) mkOption mkEnableOption types doRename;
+    # in
+    # {
+    #   options = {
+    #
+    #     # Old service
+    #     services.foo.enable = mkEnableOption "foo";
+    #
+    #     # New multi-instance service
+    #     services.foos = mkOption {
+    #       type = types.attrsOf (types.submodule …);
+    #     };
+    #   };
+    #   imports = [
+    #     (doRename {
+    #       from = [ "services" "foo" "bar" ];
+    #       to = [ "services" "foos" "" "bar" ];
+    #       visible = true;
+    #       warn = false;
+    #       use = x: x;
+    #       withPriority = true;
+    #       # Only define services.foos."" if needed. (It's not just about `bar`)
+    #       condition = config.services.foo.enable;
+    #     })
+    #   ];
+    # }
+    # ```
+    condition ? true
+  }:
     { config, options, ... }:
     let
       fromOpt = getAttrFromPath from options;