FIX: Broken GitHub folder onebox logic (#15612)

1. `html_doc.css('.Box.md')` always returns a truthy value (e.g. `[]`) so the second branch of the if-elsif never ran
2. `node&.css('text()')` was invalid code that would raise an error
3. Matching on h3 elements is no longer correct with the current html structure returned by GitHub

lib/onebox/engine/github_folder_onebox.rb

@@ -26,14 +26,10 @@ module Onebox
         if fragment
           fragment = Addressable::URI.unencode(fragment)
 
-          if html_doc.css('.Box.md')
-            # For links to markdown docs
+          # For links to markdown and rdoc
+          if html_doc.css(".Box.md, .Box.rdoc").present?
             node = html_doc.css('a.anchor').find { |n| n['href'] == "##{fragment}" }
             subtitle = node&.parent&.text
-          elsif html_doc.css('.Box.rdoc')
-            # For links to rdoc docs
-            node = html_doc.css('h3').find { |n| n['id'] == "user-content-#{fragment.downcase}" }
-            subtitle = node&.css('text()')&.first&.text
           end
 
           title = "#{title} - #{subtitle}" if subtitle

spec/lib/onebox/engine/github_folder_onebox_spec.rb

@@ -41,4 +41,17 @@ describe Onebox::Engine::GithubFolderOnebox do
       expect(@onebox.to_html).to include("<a href=\"https://github.com/discourse/discourse#setting-up-discourse\" target=\"_blank\" rel=\"noopener\">discourse/discourse - Setting up Discourse</a>")
     end
   end
+
+  context 'with rdoc fragments' do
+    before do
+      @link = "https://github.com/ruby/rdoc#description-"
+      @uri = "https://github.com/ruby/rdoc"
+      stub_request(:get, @uri).to_return(status: 200, body: onebox_response("githubfolder-rdoc-root"))
+      @onebox = described_class.new(@link)
+    end
+
+    it "extracts subtitles when linking to docs" do
+      expect(@onebox.to_html).to include("<a href=\"https://github.com/ruby/rdoc#description-\" target=\"_blank\" rel=\"noopener\">GitHub - ruby/rdoc: RDoc produces HTML and online documentation for Ruby... - Description¶ ↑</a>")
+    end
+  end
 end