docs: Expand documentation for the tickpolicy timer attribute

The current documentation is fairly terse and not easy to decode
for someone who's not intimately familiar with the inner workings
of timer devices. Expand on it by providing a somewhat verbose
description of what behavior each policy will result in, as seen
from both the guest OS and host point of view.

This is lifted directly from QEMU commit

commit 2a7d957596786404c4ed16b089273de95a9580ad
Author: Andrea Bolognani <abologna@redhat.com>
Date: Tue Feb 11 19:37:44 2020 +0100

qapi: Expand documentation for LostTickPolicy

v4.2.0-1442-g2a7d957596

The original text also matched word for word the documentation
found in QEMU.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>