devices: document interrupt supression.
devices: document interrupt supression.

There's some ring related wrapping math for interrupt suppression
(aka VIRTIO_F_EVENT_IDX) that had some documentation, but was still a little
tricky to follow. This CL adds some diagrams & further details.

BUG=none
TEST=n/a

Change-Id: I591661a676eb6506fbfea2afa20da28aaa1722f9
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3689972
Reviewed-by: Keiichi Watanabe <815e4e8f7c92a23424d7c3f51ead622bcedbc720@chromium.org>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Commit-Queue: Noah Gold <2d4550d8af93bfc6e6fbe1fb1b0cc97a9ee7c3ce@google.com>

google/crosvm,google/crosvm,google/crosvm,google/crosvm