clarify that relay request must have an established session

i2p2www/pages/site/docs/spec/ssu.html

@@ -1,7 +1,7 @@
 {% extends "global/layout.html" %}
 {% block title %}SSU Protocol Specification{% endblock %}
-{% block lastupdated %}October 2014{% endblock %}
-{% block accuratefor %}0.9.16{% endblock %}
+{% block lastupdated %}February 2015{% endblock %}
+{% block accuratefor %}0.9.18{% endblock %}
 {% block content %}
 
 <p>
@@ -572,7 +572,7 @@ This is the first message sent from Alice to Bob to request an introduction to C
     <td>Alice to Bob</td></tr>
 <tr><td align="right" valign="top"><b>Data:</b></td>
     <td><ul>
-        <li>4 byte relay (introduction) tag, nonzero</li>
+        <li>4 byte relay (introduction) tag, nonzero, as received by Alice in the Session Created message from Bob</li>
         <li>1 byte IP address size</li>
         <li>that many byte representation of Alice's IP address</li>
         <li>2 byte port number (of Alice)</li>
@@ -635,6 +635,9 @@ There are no plans to implement relaying for IPv6.
 Prior to release 0.9.12, Bob's intro key was always used.
 As of release 0.9.12, the session key is used if there is an established session
 between Alice and Bob.
+In practice, there must be an established session, as Alice will only get the
+nonce (introduction tag) from the session created message, and
+Bob will mark the introduction tag invalid once the session is destroyed.
 </li></ul>