Update PROPERTIES and ENV_PROPERTIES of user guide.
Update PROPERTIES and ENV_PROPERTIES of user guide.

Made clearer how to use properties as a proto message.

R=martiniss@google.com

Change-Id: I1bea68c23ba073c8e7b98940c96b9e016f24afc8
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/recipes-py/+/1661165
Commit-Queue: Debrian Figueroa <9840c73f2a8451542dfc1019c9a9885713dad032@google.com>
Reviewed-by: Garrett Beaty <a0767768af30bcbe01b6de5024355c539362a830@chromium.org>
Reviewed-by: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>

luci/recipes-py,luci/recipes-py