[autoroller] Update doc on tbr_emails field.
[autoroller] Update doc on tbr_emails field.

This is a much more sensible behavior requested by agable@.

R=agable@chromium.org, tandrii@chromium.org, yiwzhang@google.com

Change-Id: I2132f168a0dd713d910d64e9dde8e5815ed96af4
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/recipes-py/+/2176894
Auto-Submit: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>
Commit-Queue: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@google.com>
Reviewed-by: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@google.com>

luci/recipes-py,luci/recipes-py