Add tooltip for instance delivery warnings (#17854)

app/views/admin/instances/_instance.html.haml

 .directory__tag
   = link_to admin_instance_path(instance) do
     %h4
-      = fa_icon 'warning fw' if instance.failing?
+      = fa_icon 'warning fw', title: t('admin.instances.availability.warning') if instance.failing?
       = instance.domain
 
       %small

config/locales/en.yml

@@ -490,6 +490,7 @@ en:
           other: Failed attempts on %{count} different days.
         no_failures_recorded: No failures on record.
         title: Availability
+        warning: The last attempt to connect to this server has been unsuccessful
       back_to_all: All
       back_to_limited: Limited
       back_to_warning: Warning