add link to documentation from squash commits option in MR widget

app/assets/stylesheets/pages/merge_requests.scss

@@ -65,6 +65,15 @@
       .merge-param-checkbox {
         margin: 0;
       }
+
+      a .fa-question-circle {
+        color: $gl-text-color-secondary;
+
+        &:hover,
+        &:focus {
+          color: $link-hover-color;
+        }
+      }
     }
   }
 

app/views/projects/merge_requests/widget/open/_accept.html.haml

@@ -43,6 +43,7 @@
           = hidden_field_tag :squash, '0', id: nil
           = check_box_tag :squash, '1', @merge_request.squash
           Squash commits
+          = link_to icon('question-circle'), help_page_path('user/project/merge_requests/squash_before_merge'), title: 'Learn more', data: {toggle: 'tooltip', placement: 'bottom', container: 'body'}
 
       .accept-control
         - if @project.merge_requests_ff_only_enabled