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.
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