[machine] Update docs on how data flows.
[machine] Update docs on how data flows.

Change-Id: I7d1131695a9e9ffa794c7726a0d8e51c2c6c9bdf
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/548217
Reviewed-by: Erik Rose <042ad22d8352d59a3d816dac710aebe65456f16e@google.com>
Commit-Queue: Joe Gregorio <b9cf2471ff6d33504cafe8e6f356b732b2cadced@google.com>

google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot