Add comment regarding vars being required in blocks.

Documentation block syntax requires that at least one var be specified.

This limitation should be removed but for now add a comment to describe why a bogus var is defined.

doc/xml/user-guide.xml

@@ -1999,7 +1999,8 @@
             <title>Setup Passwordless SSH</title>
 
             <block id="setup-ssh-intro">
-                <block-variable-replace key="bogus">bogus !!!</block-variable-replace>
+                <!-- ??? Bogus variable is set because the syntax currently requires at least one -->
+                <block-variable-replace key="bogus"></block-variable-replace>
             </block>
 
             <execute-list host="{[host-repo1]}">
@@ -2330,7 +2331,8 @@
             <title>Setup Passwordless SSH</title>
 
             <block id="setup-ssh-intro">
-                <block-variable-replace key="bogus">bogus !!!</block-variable-replace>
+                <!-- ??? Bogus variable is set because the syntax currently requires at least one -->
+                <block-variable-replace key="bogus"></block-variable-replace>
             </block>
 
             <block id="setup-ssh">