Check featureCompatibilityVersion on all replica set members before replacing binaries during upgrade

In the 3.6 downgrade instructions for replica sets, users are directed to check that the featureCompatibilityVersion is 3.4 on each member of the replica set before replacing the binaries with version 3.4. Likewise, when upgrading to 3.6, users should be directed to check that the featureCompatibilityVersion is 3.4 on each member of the replica set before replacing the binaries with version 3.6. Similarly for replica set members in a sharded cluster.