Improve merge request description placeholder

Changes MR form description placeholder title into: 'Describe the goal
of the changes and what reviewers should be aware of.'

app/views/shared/form_elements/_description.html.haml

@@ -2,6 +2,7 @@
 - model = local_assigns.fetch(:model)
 
 - form = local_assigns.fetch(:form)
+- placeholder = model.is_a?(MergeRequest) ? _('Describe the goal of the changes and what reviewers should be aware of.') : _('Write a comment or drag your files here…')
 - supports_quick_actions = model.new_record?
 
 - if supports_quick_actions
@@ -16,7 +17,7 @@
     = render layout: 'projects/md_preview', locals: { url: preview_url, referenced_users: true } do
       = render 'projects/zen', f: form, attr: :description,
                                classes: 'note-textarea qa-issuable-form-description rspec-issuable-form-description',
-                               placeholder: "Write a comment or drag your files here…",
+                               placeholder: placeholder,
                                supports_quick_actions: supports_quick_actions
       = render 'shared/notes/hints', supports_quick_actions: supports_quick_actions
       .clearfix

changelogs/unreleased/30616-improve-merge-request-description-placeholder.yml

+---
+title: Improve merge request description placeholder
+merge_request: 20032
+author: Jacopo Beschi @jacopo-beschi
+type: changed

locale/gitlab.pot

@@ -5641,6 +5641,9 @@ msgstr ""
 msgid "Descending"
 msgstr ""
 
+msgid "Describe the goal of the changes and what reviewers should be aware of."
+msgstr ""
+
 msgid "Description"
 msgstr ""
 

spec/features/merge_request/user_creates_merge_request_spec.rb

@@ -25,6 +25,11 @@ describe "User creates a merge request", :js do
 
     click_button("Compare branches")
 
+    page.within('.merge-request-form') do
+      expect(page.find('#merge_request_title')['placeholder']).to eq 'Title'
+      expect(page.find('#merge_request_description')['placeholder']).to eq 'Describe the goal of the changes and what reviewers should be aware of.'
+    end
+
     fill_in("Title", with: title)
     click_button("Submit merge request")