Description

For https://docs.mongodb.com/manual/reference/command/setFeatureCompatibilityVersion/#dbcmd.setFeatureCompatibilityVersion

 In SERVER-37049, Tess wrote:

Arbiters always have the downgrade value of FCV, regardless of the FCV of the replica set. So if the arbiter has binary version 3.6, it will always have FCV=3.4. They never learn the FCV, since they do not replicate the admin.system.version collection. Arbiters' behavior is not allowed to depend on FCV for this reason.

We should add a small section titled "Arbiters" to explain that this is expected behaviour since it can unexpected.

Scope of changes

Impact to Other Docs

MVP (Work and Date)

Resources (Scope or Design Docs, Invision, etc.)