Use absolute URL for scoped label doc link

We need an absolute URL here because relative links in Markdown are
turned into repository file links by RepositoryLinkFilter.

ee/app/helpers/ee/labels_helper.rb

@@ -74,7 +74,7 @@ module EE
     private
 
     def scoped_labels_doc_link
-      help_url = ::Gitlab::Routing.url_helpers.help_page_path('user/project/labels.md', anchor: 'scoped-labels-premium')
+      help_url = ::Gitlab::Routing.url_helpers.help_page_url('user/project/labels.md', anchor: 'scoped-labels-premium')
 
       %(<a href="#{help_url}" class="gl-link gl-label-icon" target="_blank" rel="noopener"><i class="fa fa-question-circle"></i></a>).html_safe
     end

ee/changelogs/unreleased/fix-scoped-label-doc-link-in-markdown.yml

+---
+title: Fix invalid scoped label documentation URL when rendered in Markdown
+merge_request: 28268
+author:
+type: fixed

ee/spec/lib/banzai/filter/label_reference_filter_spec.rb

@@ -14,16 +14,18 @@ describe Banzai::Filter::LabelReferenceFilter do
       stub_licensed_features(scoped_labels: true)
     end
 
-    it 'includes link to scoped documentation' do
+    it 'renders scoped label with link to documentation' do
       doc = reference_filter("See #{scoped_label.to_reference}")
 
-      expect(doc.to_html).to match(%r(<span.+><a.+><span.+>#{scoped_label.scoped_label_key}</span><span.+>#{scoped_label.scoped_label_value}</span></a><a.+>.*question-circle.*</a></span>))
+      expect(doc.css('.gl-label-scoped .gl-label-text').map(&:text)).to eq([scoped_label.scoped_label_key, scoped_label.scoped_label_value])
+      expect(doc.at_css('a.gl-label-icon')['href']).to eq('http://localhost/help/user/project/labels.md#scoped-labels-premium')
     end
 
-    it 'does not include link to scoped documentation for common labels' do
+    it 'renders common label' do
       doc = reference_filter("See #{label.to_reference}")
 
-      expect(doc.to_html).to match(%r(<span.+><a.+><span.+>#{label.name}</span></a></span>$))
+      expect(doc.css('.gl-label .gl-label-text').map(&:text)).to eq([label.name])
+      expect(doc.at_css('a.gl-label-icon')).to be_nil
     end
   end
 
@@ -32,10 +34,11 @@ describe Banzai::Filter::LabelReferenceFilter do
       stub_licensed_features(scoped_labels: false)
     end
 
-    it 'renders label as a common label' do
+    it 'renders scoped label as a common label' do
       doc = reference_filter("See #{scoped_label.to_reference}")
 
-      expect(doc.to_html).to match(%r(<span.+><a.+><span.+>#{scoped_label.name}</span></a></span>$))
+      expect(doc.css('.gl-label .gl-label-text').map(&:text)).to eq([scoped_label.name])
+      expect(doc.at_css('a.gl-label-icon')).to be_nil
     end
   end
 end