Recipes user guide: document `./recipes.py debug`
Recipes user guide: document `./recipes.py debug`

BUG=None
TEST=view in gitiles

Change-Id: I844e0f82074255258bf062dd169600648418b746
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/recipes-py/+/3820981
Commit-Queue: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>
Reviewed-by: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>
Auto-Submit: Greg Edelston <57c05a6c437bf6d235ddca921bf7f9040fa804d5@google.com>

luci/recipes-py,luci/recipes-py