FIX: do not generate multiple detail blocks when the selected input (#5290)

consists of multiple lines

plugins/discourse-details/assets/javascripts/initializers/apply-details.js.es6

@@ -18,7 +18,7 @@ function initializeDetails(api) {
           "\n" + `[details="${I18n.t("composer.details_title")}"]` + "\n",
           "\n[/details]\n",
           "details_text",
-          { multiline: true }
+          { multiline: false }
         );
         this.set('optionsVisible', false);
       }