[warning] add user guide for warning feature in recipe engine
[warning] add user guide for warning feature in recipe engine

R=iannucci

Change-Id: Id66b832a7215bbabc4811d2d0eecba8091ff28fb
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/recipes-py/+/2144702
Commit-Queue: Yiwei Zhang <50b2a565e2e78c292794832469a30ce5abc9959c@google.com>
Reviewed-by: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>
Auto-Submit: Yiwei Zhang <50b2a565e2e78c292794832469a30ce5abc9959c@google.com>

luci/recipes-py,luci/recipes-py