docs/memory.txt: Clarify and expand priority/overlap documentation

The documentation of how overlapping memory regions behave and how
the priority system works was rather brief, and confusion about
priorities seems to be quite common for developers trying to understand
how the memory region system works, so expand and clarify it.
This includes a worked example with overlaps, documentation of the
behaviour when an overlapped container has "holes", and mention
that it's valid for a region to have both MMIO callbacks and
subregions (and how this interacts with priorities when it does).

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Message-id: 1381848154-31602-1-git-send-email-peter.maydell@linaro.org
Signed-off-by: Anthony Liguori <aliguori@amazon.com>
