docs: document missing POWERDOWN event

The POWERDOWN event was first documented in 0aab9ec.  But since
dfab489 later restored this file to the state prior to qmp events,
and we never documented it in the past, anyone using this file
instead of qapi will miss out on this event.  Tweak the existing
wording of SHUTDOWN to match 8432183, and make the difference
between the two events apparent.

* docs/qmp/qmp-events.txt (POWERDOWN): Add.
(SHUTDOWN): Tweak.

Signed-off-by: Eric Blake <[email protected]>
Reviewed-by: Markus Armbruster <[email protected]>
Reviewed-by: Wenchao Xia <[email protected]>
Signed-off-by: Luiz Capitulino <[email protected]>

docs/qmp/qmp-events.txt

@@ -243,6 +243,19 @@ Data:
   "timestamp": { "seconds": 1368697518, "microseconds": 326866 } }
 }
 
+POWERDOWN
+---------
+
+Emitted when the Virtual Machine is powered down through the power
+control system, such as via ACPI.
+
+Data: None.
+
+Example:
+
+{ "event": "POWERDOWN",
+    "timestamp": { "seconds": 1267040730, "microseconds": 682951 } }
+
 QUORUM_FAILURE
 --------------
 
@@ -325,7 +338,8 @@ Example:
 SHUTDOWN
 --------
 
-Emitted when the Virtual Machine is powered down.
+Emitted when the Virtual Machine has shut down, indicating that qemu
+is about to exit.
 
 Data: None.