templatekw/help: document the {parents} keyword The {parents} keyword does not appear in the generated documentation for templates because it is added by `changeset_templater` (and this is because its behavior depends on `ui`, so it can't be defined as a normal template keyword; see comments in `changeset_templater._show()`). Add it to the documentation synthetically by creating a stub documentation function. Test plan: built the docs and examined the man page to verify that this keyword is now documented. I'm not sure how to test the i18n extraction part, but assume it will just work given that this patch doesn't do anything too crazy.