hmp: Add help information for watchdog action: inject-nmi

virsh qemu-monitor-command --hmp help information of
watchdog_action missing inject-nmi which already supported
in Commit 795dc6e4

Resolves: qemu-project/qemu#2247


Signed-off-by: Dayu Liu <liu.dayu@zte.com.cn>
Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
[Mjt: decode and word-wrap commit message and add Resolves: tag]

hmp-commands.hx

@@ -1412,7 +1412,7 @@ ERST
     {
         .name       = "watchdog_action",
         .args_type  = "action:s",
-        .params     = "[reset|shutdown|poweroff|pause|debug|none]",
+        .params     = "[reset|shutdown|poweroff|pause|debug|none|inject-nmi]",
         .help       = "change watchdog action",
         .cmd        = hmp_watchdog_action,
         .command_completion = watchdog_action_completion,