ivshmem: document a potential crash in rte_ring

ivshmem: document a potential crash in rte_ring

Commit 4768c475 added a pointer to the memzone in rte_ring. However,
all memzones are residing in local mem_config, therefore accessing
the memzone pointer inside the guest in an IVSHMEM-shared rte_ring
will cause segmentation fault. This issue is unlikely to ever get
fixed, as this would require lots of changes for very little benefit,
therefore we're documenting this limitation instead.

Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>