This is for the document write concern majority:

Requests acknowledgment that write operations have propagated to the majority of voting nodes [2], including the primary.

Currently we only mention that "majority" is calculated by the majority of voting nodes. However, this doesn't cover the scenario when the majority of voting nodes are arbiters, like Primary-Arbiter-Arbiter (thought this is not a recommended deployment).

It would be nice to clarify that it calculated by the majority of voting nodes or all non-arbiter voting nodes if the majority of voting nodes are arbiters. all non-arbiter voting nodes if at least half of voting nodes are arbiters.

https://github.com/mongodb/mongo/blob/master/src/mongo/db/repl/replica_set_config.cpp#L688-L699

Edit: updated "all non-arbiter voting nodes if the majority of voting nodes are arbiters." to "all non-arbiter voting nodes if at least half of voting nodes are arbiters"