Add hint to letter contact details input

Important for users to know about this limit.

Think it got dropped in all the merging somewhere.

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,
+        hint='10 lines maximum',
         width='1-1',
         rows=10
       ) }}