Studio Section Highlights: add doc and preview text to modal

cms/templates/js/highlights-editor.underscore

@@ -1,5 +1,33 @@
 <form>
 <h3 class="modal-section-title" id="highlights_label"><%- gettext('Section Highlights') %></h3>
+    <div class="highlights-info">
+
+<% if (highlights_preview_only) { %>
+    <p><b>
+    <%- gettext('This feature is currently in testing. Course teams can enter highlights, but learners will not receive email messages.') %>
+    </b></p>
+<% } %>
+
+<%- gettext('Enter 3-5 highlights to include in the email message that learners receive for this section (250 character limit).') %>
+
+<p>
+<br>
+<% // xss-lint: disable=underscore-not-escaped %>
+<%= edx.HtmlUtils.interpolateHtml(
+    gettext(
+        'For more information and an example of the email template, ' +
+        'read our {linkStart}documentation{linkEnd}.'
+    ),
+    {
+        linkStart: edx.HtmlUtils.HTML('<a href="' + highlights_doc_url + '">'),
+        linkEnd: edx.HtmlUtils.HTML('</a>')
+    }
+) %>
+</p>
+</div>
+
+<hr>
+
 <div class="modal-section-content block-highlights">
     <div role="group" class="list-fields" aria-labelledby="highlights_label">
         <%