Added hostkey field to hostswithclass

Signed-off-by: Victor Moene <victor.moene@northern.tech>

reference/functions/hostswithclass.markdown

@@ -12,13 +12,13 @@ published: true
 `field` of hosts on which `classs` is set.
 
 On CFEngine Enterprise, this function can be used to return a list of
-hostnames or ip-addresses of hosts that have a given class.
+hostnames, ip-addresses or public key shas of hosts that have a given class.
 
 **Note:** This function only works locally on the hub, but allows the hub to construct custom
 configuration files for (classes of) hosts. Hosts are selected based on the
 classes set during the most recently collected agent run.
 
-[%CFEngine_function_attributes(class, field)%]
+[%CFEngine_function_attributes(class, field, hostkey)%]
 
 **Example:**