Add documentation for placeholders.
Add documentation for placeholders.

Change-Id: Icc762c0a8de1e92440e8cf444d2203f3eac68c9d
Reviewed-on: https://chromium-review.googlesource.com/1234676
Commit-Queue: Erik Chen <b51d12b71fa2f8cb17021720ad5ee62fe09eb9e9@chromium.org>
Reviewed-by: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>

luci/recipes-py,luci/recipes-py