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/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