Rephrase burndown chart warning

app/helpers/milestones_helper.rb

@@ -121,13 +121,13 @@ module MilestonesHelper
 
     message =
       if burndown.empty?
-        "The burndown chart can’t be shown, as all milestone issues were closed before the GitLab 9.1 update. "
+        "The burndown chart can’t be shown, as all issues assigned to this milestone were closed on an older GitLab version before data was recorded. "
       elsif !burndown.accurate?
-        "Some issues can’t be shown in the burndown chart, as they were closed before the GitLab 9.1 update. "
+        "Some issues can’t be shown in the burndown chart, as they were closed on an older GitLab version before data was recorded. "
       end
 
     if message
-      message += link_to "More information.", help_page_path('user/project/milestones/index', anchor: 'burndown-charts'), class: 'burndown-docs-link'
+      message += link_to "Find out about burndown charts.", help_page_path('user/project/milestones/index', anchor: 'burndown-charts'), class: 'burndown-docs-link'
 
       content_tag(:div, message.html_safe, id: "data-warning", class: "settings-message prepend-top-20")
     end

doc/user/project/milestones/index.md

@@ -44,8 +44,12 @@ special options available when filtering by milestone:
 
 ## Burndown charts
 
-> [Introduced][ee-1540] in GitLab Enterprise Edition 9.1 and is available for
+>**Notes:**
+- [Introduced][ee-1540] in GitLab Enterprise Edition 9.1 and is available for
   [Enterprise Edition Starter][ee] users.
+- Closed or reopened issues prior to GitLab 9.1 won't have a `closed_at`
+  value, so the burndown chart considers them as closed on the milestone
+  `start_date`. In that case, a warning will be displayed.
 
 A burndown chart is available for every project milestone that has a set start
 date and a set due date and is located on the project's milestone page.
@@ -64,11 +68,5 @@ cumulative value.
 
 ![burndown chart](img/burndown_chart.png)
 
-Closed or reopened issues prior to GitLab 9.1 version won't have a `closed_at`
-value, so the burndown chart considers it as closed on the milestone `start_date`.
-In that case, a warning will be displayed.
-
-![burndown chart warning](img/burndown_warning.png)
-
 [ee-1540]: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1540
 [ee]: https://about.gitlab.com/gitlab-ee