Explain about text message prefixes when editing

Previously this lead to 2 support tickets because the user didn't
understand why their messages was being split into 2 fragments. We
tried modifying the message about charges, but that made it more
complicated. Adding a hint should hopefully be enough.

app/templates/views/edit-sms-template.html

@@ -15,6 +15,10 @@
       back_link=url_for('main.view_template', service_id=current_service.id, template_id=template.id) if template else url_for('main.choose_template', service_id=current_service.id, template_folder_id=template_folder_id)
     ) }}
 
+    {% if current_service.prefix_sms %}
+      {% set content_hint = 'All messages will start with your service name' %}
+    {% endif %}
+
     {% call form_wrapper() %}
       <div class="govuk-grid-row">
         <div class="govuk-grid-column-two-thirds">
@@ -26,6 +30,7 @@
             form.template_content,
             highlight_placeholders=True,
             width='1-1',
+            hint=content_hint,
             rows=5,
             extra_form_group_classes='govuk-!-margin-bottom-2'
           ) }}