Add some explaining to the SMS sender page

app/main/forms.py

@@ -331,13 +331,17 @@ class ServiceReplyToEmailFrom(Form):
 
 
 class ServiceSmsSender(Form):
-    sms_sender = StringField('', validators=[Length(max=11,
+    sms_sender = StringField(
-                             message="Text message sender can't be longer than 11 characters")])
+        'Text message sender',
+        validators=[
+            Length(max=11, message="Enter fewer than 11 characters")
+        ]
+    )
 
     def validate_sms_sender(form, field):
         import re
         if field.data and not re.match('^[a-zA-Z0-9\s]+$', field.data):
-            raise ValidationError('Text message sender can only contain alpha-numeric characters')
+            raise ValidationError('Use letters and numbers only')
 
 
 class ServiceBrandingOrg(Form):

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',

tests/app/main/test_validators.py

@@ -137,8 +137,8 @@ def test_sms_sender_form_validation(app_, mock_get_user_by_email):
 
         form.sms_sender.data = 'morethanelevenchars'
         form.validate()
-        assert "Text message sender can't be longer than 11 characters" == form.errors['sms_sender'][0]
+        assert "Enter fewer than 11 characters" == form.errors['sms_sender'][0]
 
         form.sms_sender.data = '###########'
         form.validate()
-        assert 'Text message sender can only contain alpha-numeric characters' == form.errors['sms_sender'][0]
+        assert 'Use letters and numbers only' == form.errors['sms_sender'][0]