https://www.mongodb.com/docs/manual/core/aggregation-pipeline-optimization/#indexes

In this section, I was expecting to find the list of stages that completely prevent the use of indexes further down the pipeline. 

For example I was convinced that $project and $unset were preventing any indexes from being used based on the documentation but I was surprised to see that these 2 pipelines can actually both use an index on {name:1}.

db.c.explain(true).aggregate([{"$project": {"age":0}},{"$match": {name:"Max"}}])
db.c.explain(true).aggregate([{"$unset": "age"},{"$match": {name:"Max"}}])

Can we improve the doc to be more precise?
Thanks!