Add discussion of challenge/exchange lifetime.

index.html

@@ -857,7 +857,11 @@ <h4>Get Workflow Configuration</h4>
         <div class="api-detail"
           data-api-endpoint="get /workflows/{localWorkflowId}"></div>
       </section>
-
+      <p>
+There is a `ttl` or time-to-live property associated with exchanges created using the /workflows/{localWorkflowId}/exchanges
+endpoint. This impacts the lifetime of challenges associated with such an exchange: if a
+challenge is bound to an exchange, the lifetime of that challenge is the `ttl` of the exchange.
+      </p>
       <section>
         <h4>Create Exchange</h4>
         <p>