buffet: Add some documentation to the Command interface

BUG=None
TEST=compiles

Change-Id: I8f6f55a774306dc4f76d3339f38eb43b6f8d39f4
Reviewed-on: https://chromium-review.googlesource.com/257040
Tested-by: Christopher Wiley <wiley@chromium.org>
Reviewed-by: David Zeuthen <zeuthen@chromium.org>
Commit-Queue: Christopher Wiley <wiley@chromium.org>
Reviewed-by: Alex Vakulenko <avakulenko@chromium.org>