Add some explaining to the SMS sender page

app/templates/views/service-settings/set-sms-sender.html

@@ -10,12 +10,27 @@
 
   <div class="grid-row">
     <div class="column-three-quarters">
-      <h1 class="heading-large">Set text message sender name</h1>
+      <h1 class="heading-large">Text message sender</h1>
+      <p>
+        This appears instead of a phone number when a user receives a
+        text message from your service.
+      <p>
+        If you leave this blank:
+      </p>
+      <ul class="list list-bullet">
+        <li>
+          your messages will be sent from 40604 (a shortcode that’s
+          reserved for government use)
+        </li>
+        <li>
+          each message will begin with ‘{{ current_service.name }}:’
+        </li>
+      </ul>
       <form method="post">
         {{ textbox(
           form.sms_sender,
           width='1-1',
-          safe_error_message=True
+          hint='Up to 11 characters, letters, numbers and spaces only'
         ) }}
         {{ page_footer(
           'Save',