This does two things:
1. It increases the contrast of the signatures
2. It ensures that there's some spacing when there's two signature
elements back-to-back (which happens when documenting structs, for
instance), making it easier to tell things apart.
Reviewed-by: Jani Nikula <jani@nikula.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24507>