devices: extend IrqLevelEvent documentation
devices: extend IrqLevelEvent documentation

Describe the resample event and how it is meant to be used by a device.

BUG=b:243465984
TEST=tools/cargo-doc

Change-Id: I0a2ad7b5593e0222e69c932fed20499f6d2fccbf
Reviewed-on: https://chromium-review.googlesource.com/c/crosvm/crosvm/+/3892525
Reviewed-by: Dennis Kempin <cd09796fb571bec2782819dbfd33307f65b1c778@google.com>
Commit-Queue: Daniel Verkamp <72bc170b46ec491f7bdd4359a1c0bfed274de40c@chromium.org>

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