[step|docs] Improve the nest docstring a bit.
[step|docs] Improve the nest docstring a bit.

R=martiniss@chromium.org, phajdan.jr@chromium.org, tansell@chromium.org
BUG=

Review-Url: https://codereview.chromium.org/2672873003

luci/recipes-py,luci/recipes-py