Update trial mode page to talk about letters

The meaning of trial mode has evolved now that we’re adding letters:
- the existing conditions only apply to texts and emails
- we need to explain what trial mode means for letters

app/templates/views/trial-mode.html

@@ -17,7 +17,7 @@
     </p>
     <ul class="list list-bullet">
       <li>
-        you can only send messages to yourself
+        you can only send text messages and emails to yourself
       </li>
       <li>
         you can add people to
@@ -26,10 +26,13 @@
         {% else %}
           your team,
         {% endif %}
-        then you can send messages to them too
+        then you can send text messages and emails to them too
       </li>
       <li>
-        you can only send 50 messages per day
+        you can only send 50 text messages or emails per day
+      </li>
+      <li>
+        you can’t send any letters
       </li>
     </ul>