doc: Complement HIPC document for RPMsg frame drop

The RPMsg frame may be dropped in the virtio-rpmsg driver silently
when traffic exceeds the performance limitation.

Issue-Id: SCM-5876
Signed-off-by: Yanqin Wei <Yanqin.Wei@arm.com>
Change-Id: Iedd2908d6d099a1aea7666e1ac7575f273cf8b15