Restore accidentally removed documentation for `Tanshuku::Configurati…

…on.default_url_options`

They were unintentionally removed in commit ca93377.

lib/tanshuku/configuration.rb

@@ -43,6 +43,22 @@ def self.call(exception:, original_url:)
     end
 
     include ActiveModel::Attributes
+
+    # @!attribute [rw] default_url_options
+    #   Default URL options for Rails’ +url_for+. Defaults to +{}+.
+    #
+    #   @return [Hash]
+    #   @return [void] If you set an invalid object.
+    #
+    #   @note
+    #     The example below means that the configured host and protocol are used. Shortened URLs will be like
+    #     +https://example.com/t/abcdefghij0123456789+.
+    #
+    #   @example
+    #     # config/initializers/tanshuku.rb
+    #     Tanshuku.configure do |config|
+    #       config.default_url_options = { host: "example.com", protocol: :https }
+    #     end
     attribute :default_url_options, default: {}
 
     # @!attribute [rw] max_url_length