Thanks for taking care of this! I assume you've verified that the documentation now looks as it should be, so I'm not checking the documentation myself.

The issue title is misleading, so I'm updating it for the purpose of the change log for the next release. Phrased like this, we would typically mean it as the *action* that was done, but you meant it exactly the other way around, and also it doesn't reflect what was actually done in the end.

Esther reviewed the PR and we merged it.

I made a small PR (~40 lines changed and most of them are moved text) for the issue that is ready for review
https://github.com/aibasel/downward/pull/289

I moved the part of the synopsis that is replaced later on to a 'note' instead.
Additionally, I added assertions to prevent such documentation bugs in the future.

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.