Move exclude_rel_nofollow_domains setting to the Required setting and improve copy.

config/locales/server.en.yml

@@ -768,7 +768,7 @@ en:
     show_subcategory_list: "Show subcategory list instead of topic list when entering a category."
     fixed_category_positions: "If checked, you will be able to arrange categories into a fixed order. If unchecked, categories are listed in order of activity."
     add_rel_nofollow_to_user_content: "Add rel nofollow to all submitted user content, except for internal links (including parent domains). If you change this, you must rebake all posts with: \"rake posts:rebake\""
-    exclude_rel_nofollow_domains: "A pipe-delimited list of domains where nofollow is not added (tld.com will automatically allow sub.tld.com as well)"
+    exclude_rel_nofollow_domains: "A list of domains where nofollow should not be added to links. tld.com will automatically allow sub.tld.com as well. As a minimum, you should add the top-level domain of this site to help web crawlers find all content. If other parts of your website are at other domains, add those too."
 
     post_excerpt_maxlength: "Maximum length of a post excerpt / summary."
     post_onebox_maxlength: "Maximum length of a oneboxed Discourse post in characters."

config/site_settings.yml

@@ -51,6 +51,9 @@ required:
     client: true
     default: '/images/default-favicon.ico'
   apple_touch_icon_url: '/images/default-apple-touch-icon.png'
+  exclude_rel_nofollow_domains:
+    default: ''
+    type: list
 
 basic:
   default_locale:
@@ -636,9 +639,6 @@ onebox:
 
 spam:
   add_rel_nofollow_to_user_content: true
-  exclude_rel_nofollow_domains:
-    default: ''
-    type: list
   flags_required_to_hide_post: 3
   cooldown_minutes_after_hiding_posts: 10
   num_flags_to_block_new_user: 3