doc: update 'asa.rst' for 2.3 release
update fixed security issue for 2.3 release.
Security Advisory
#################
+Addressed in ACRN v2.3
+************************
+
+We recommend that all developers upgrade to this v2.3 release (or later), which
+addresses the following security issue that was discovered in previous releases:
+
+------
+
+- NULL Pointer Dereference in ``devicemodel\hw\pci\virtio\virtio_mei.c``
+ ``vmei_proc_tx()`` function tries to find the ``iov_base`` by calling
+ function ``paddr_guest2host()``, which may return NULL (the ``vd``
+ struct control by the User VM OS). There is a use of ``iov_base``
+ afterward that can cause a NULL pointer dereference (CVE-2020-28346).
+
+ **Affected Release:** v2.2 and earlier.
+
Addressed in ACRN v2.1
************************
