[doc] Change default to "gen".
[doc] Change default to "gen".

This is, by far, the most used 'kind' for the doc subcommand. This will
let folks regenerate `README.recipes.md` by just running `./recipes.py doc`
instead of `./recipes.py doc --kind gen`.

R=martiniss@chromium.org, a30c74fa30536fe7ea81ed6dec202e35e149e1fd@chromium.org

Change-Id: I3f6896f9ec4eb95a13eee567fa86bf4d93e7d20c
Reviewed-on: https://chromium-review.googlesource.com/c/1484515
Auto-Submit: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>
Commit-Queue: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@chromium.org>
Reviewed-by: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@chromium.org>

luci/recipes-py,luci/recipes-py