Explain text message sender

We sometimes have to do this over support tickets as part of the go-live
process; now we’re directing people to add a sender (as part of the task
list) we can explain what it is in context.

app/templates/views/service-settings/sms-senders.html

@@ -23,7 +23,7 @@
   <div class="user-list">
     {% if not sms_senders %}
       <div class="user-list-item">
-        <span class="hint">You haven’t added any sms senders yet</span>
+        <span class="hint">You haven’t added any text message senders yet</span>
       </div>
     {% endif %}
     {% for item in sms_senders %}
@@ -46,4 +46,7 @@
       </div>
     {% endfor %}
   </div>
+  <p>
+    The text message sender tells your users who the message is from.
+  </p>
 {% endblock %}