In the cc file https://github.com/aibasel/downward/blob/main/src/search/landmarks/landmark_sum_heuristic.cc
we want to add to the documentation that landmark_sum is "Formerly known as the landmark heuristic or landmark count heuristic".

This does not appear on the website 
https://www.fast-downward.org/HEAD/documentation/search/Evaluator/#landmark_sum_heuristic

The reason is that we first set the specific synopsis but later call `add_landmark_heuristic_options_to_feature(*this, "landmark_sum_heuristic");`
which writes some general synopsis about landmark heuristics i.e. it calls `feature.document_synopsis("[...]")`.

The way 'document_synopsis' is implemented in `plugins/plugin.cc`
it replaces the synopsis.
I think the correct thing would be to add to it.