docs: update documentation for memory hot unplug

Add specification about how to use memory hot unplug, and add
a flow diagram to explain memory hot unplug process.

Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Zhu Guihua <zhugh.fnst@cn.fujitsu.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

docs/memory-hotplug.txt

@@ -4,9 +4,7 @@ QEMU memory hotplug
 This document explains how to use the memory hotplug feature in QEMU,
 which is present since v2.1.0.
 
-Please, note that memory hotunplug is not supported yet. This means
-that you're able to add memory, but you're not able to remove it.
-Also, proper guest support is required for memory hotplug to work.
+Guest support is required for memory hotplug to work.
 
 Basic RAM hotplug
 -----------------
@@ -74,3 +72,22 @@ comes from regular RAM, 1GB is a 1GB hugepage page and 256MB is from
    -device pc-dimm,id=dimm1,memdev=mem1 \
    -object memory-backend-file,id=mem2,size=256M,mem-path=/mnt/hugepages-2MB \
    -device pc-dimm,id=dimm2,memdev=mem2
+
+
+RAM hot-unplug
+---------------
+
+In order to be able to hot unplug pc-dimm device, QEMU has to be told the ids
+of pc-dimm device and memory backend object. The ids were assigned when you hot
+plugged memory.
+
+Two monitor commands are used to hot unplug memory:
+
+ - "device_del": deletes a front-end pc-dimm device
+ - "object_del": deletes a memory backend object
+
+For example, assuming that the pc-dimm device with id "dimm1" exists, and its memory
+backend is "mem1", the following commands tries to remove it.
+
+  (qemu) device_del dimm1
+  (qemu) object_del mem1