[bazel] Document interaction between entrypoint and run_commands_root
[bazel] Document interaction between entrypoint and run_commands_root

Bug: skia:12506
Change-Id: I25c936a520bb9f5870845c9ce897a4a57f714c97
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/457117
Reviewed-by: Leandro Lovisolo <f558143c2568be8d55575d13f30a1fc5cb8009d5@google.com>
Commit-Queue: Leandro Lovisolo <f558143c2568be8d55575d13f30a1fc5cb8009d5@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