Add some rough documentation for config.js properties.

vimperator/content/config.js

@@ -33,7 +33,10 @@ const config = { //{{{
 
     /*** optional options, there are checked for existence and a fallback provided  ***/
     features: ["bookmarks", "hints", "history", "marks", "quickmarks", "session", "tabs", "windows"],
-    defaults: { guioptions: "rb" },
+    defaults: {
+        guioptions: "rb",
+        showtabline: 2,
+    },
 
     guioptions: {
         m: ["Menubar",      ["toolbar-menubar"]],
@@ -132,10 +135,6 @@ const config = { //{{{
         }
     },
 
-    optionDefaults: {
-        showtabline: 2,
-    },
-
     scripts: [
         "bookmarks.js",
         "tabs.js",