Document the dependency on OOB for gnt-node power

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>

man/gnt-node.rst

@@ -612,3 +612,7 @@ POWER
 This commands calls out to out-of-band management to change the power
 state of given node. With ``status`` you get the power status as reported
 by the out-of-band managment script.
+
+Note that this command will only work if the out-of-band functionality
+is configured and enabled on the cluster. If this is not the case,
+please use the **powercycle** command above.