crosvm: Document the pupr process.
crosvm: Document the pupr process.

'pupr' and 'uprev' are terms that we use quite often, add some documentation.

BUG=None
TEST=read it.

Change-Id: I888d024b0cffabd43beadce0d76a1cc3644a39e3
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3292385
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Commit-Queue: Junichi Uekawa <6bd240a6ce32463e272bb69c685a8e946b33ac05@chromium.org>
Reviewed-by: Dennis Kempin <cd09796fb571bec2782819dbfd33307f65b1c778@google.com>

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