Add a note to make the users aware that some stats groups or fields may
be missing in certain cases.
* "block.<num>.errors" - Xen only: the 'oo_req' value as
* unsigned long long.
*
+ * Note that entire stats groups or individual stat fields may be missing from
+ * the output in case they are not supported by the given hypervisor, are not
+ * applicable for the current state of the guest domain, or their retrieval
+ * was not successful.
+ *
* Using 0 for @stats returns all stats groups supported by the given
* hypervisor.
*