doc: update the 'ivshmem' HLD

Update the 'ivshmem' document to clarify the existence of two similar
mechanisms to expose this device to User VMs. One is implemented in the ACRN
Device Model and another (future) is implemented in the hypervisor.

Signed-off-by: Geoffroy Van Cutsem <geoffroy.vancutsem@intel.com>

doc/developer-guides/hld/ivshmem-hld.rst

@@ -4,14 +4,14 @@ ACRN Shared Memory Based Inter-VM Communication
 ###############################################
 
 ACRN supports inter-virtual machine communication based on a shared
-memory mechanism, The ACRN device model and hypervisor emulate a virtual
+memory mechanism. The ACRN device model or hypervisor emulate a virtual
 PCI device (called an ``ivshmem`` device) to expose the base address and
 size of this shared memory.
 
 Inter-VM Communication Overview
 *******************************
 
-.. figure:: images/ivshmem-image1.png
+.. figure:: images/ivshmem-architecture.png
    :align: center
    :name: ivshmem-architecture-overview
 
@@ -27,15 +27,19 @@ VMs.
    memory regions reserved in the hypervisor's memory space.  This solution
    would work for both pre-launched and post-launched VMs.
 
-
 ivshmem hv:
-   The ivshmem device model implements register virtualization
-   and shared memory mapping in the hypervisor (not the acrn-dm).
-   Will support notification/interrupt mechanism in the future.
+   The **ivshmem hv** implements register virtualization
+   and shared memory mapping in the ACRN hypervisor.
+   It will support notification/interrupt mechanism in the future.
+
+ivshmem dm:
+   The **ivshmem dm** implements register virtualization
+   and shared memory mapping in the ACRN Device Model (``acrn-dm``).
+   It will support notification/interrupt mechanism in the future.
 
 ivshmem server:
-   A daemon for inter-VM notification capability,
-   This is currently **not implemented**, so the inter-VM communication
+   A daemon for inter-VM notification capability, that will work with **ivshmem
+   dm**. This is currently **not implemented**, so the inter-VM communication
    doesn't support a notification mechanism.
 
 Ivshmem Device Introduction
@@ -96,7 +100,7 @@ Usage
 *****
 
 To support two post-launched VMs communicating via an ``ivshmem`` device,
-add this line as an acrn-dm boot parameter::
+add this line as an ``acrn-dm`` boot parameter::
 
   -s slot,ivshmem,shm_name,shm_size
 
@@ -112,6 +116,7 @@ where
 -  ``shm_size`` - Specify a shared memory size. The two communicating
    VMs must define the same size.
 
+.. note:: This device can be used with Real-Time VM (RTVM) as well.
 
 Inter-VM Communication Example
 ******************************
@@ -119,6 +124,8 @@ Inter-VM Communication Example
 The following example uses inter-vm communication between two Linux-based post-launched VMs (VM1 and
 VM2).
 
+.. note:: An ``ivshmem`` Windows driver exists and can be found `here <https://github.com/virtio-win/kvm-guest-drivers-windows/tree/master/ivshmem>`_
+
 1. Add a new virtual PCI device for both VMs: the device type is
    ``ivshmem``, shared memory name is ``test`` and shared memory size is
    4096 bytes. Both VMs must have the same shared memory name and size: