Explain contact details changes apply all letters

This is different to how editing most parts of a letter works, so we
should make it explicit.

app/main/forms.py

@@ -502,9 +502,7 @@ class ServiceSmsSender(Form):
 
 
 class ServiceLetterContactBlock(Form):
-    letter_contact_block = TextAreaField(
-        'How should users contact you?'
-    )
+    letter_contact_block = TextAreaField()
 
     def validate_letter_contact_block(form, field):
         line_count = field.data.strip().count('\n')

app/templates/views/service-settings/set-letter-contact-block.html

@@ -15,6 +15,7 @@
     <form method="post" class="column-half">
       {{ textbox(
         form.letter_contact_block,
+        label='How should users contact your service?<br>This applies to all the letters you send.'|safe,
         hint='10 lines maximum',
         width='1-1',
         rows=10