Clarify the distinction between local and fully qualified IDs.

This is currently only relevant to the workflows and exchanges APIs
so it is added to that section.

index.html

@@ -1128,6 +1128,12 @@ <h3>Workflows and Exchanges</h3>
       <table class="simple api-summary-table"
         data-api-path="/workflows /workflows/{localWorkflowId} /workflows/{localWorkflowId}/exchanges /workflows/{localWorkflowId}/exchanges/{localExchangeId}"></table>
 
+      <p>
+In the workflows and exchanges APIs, a "local" ID refers to an ID that is local to a service instance.
+In other words, an `exchangeId` or `workflowId` refers to a fully qualified URL
+while a `localExchangeId` or `localWorkflowId` refers to a fragment of the URL.
+      </p>
+
       <section>
         <h4>Create Workflow</h4>
         <p>