Should Fileshare be part of Cluster resource in multisubnet AAG?
I'd need to see the article where that quote is from to understand the context, but the file share will still be part of the cluster and will still be configured as the witness. 

Votes: Node1=Yes, Node2=Yes, Node3=No, File share=Yes

If you ever failover to the other data center, you'll need to create a quorum there. Maybe you'd spin up another node and file share when you failover. Those would be created in the San Jose data center. Once those are in place, add the VM to the cluster and then reconfigure the cluster to use the new file share in the San Jose DC for the witness. After failover, disable the votes for the DC data center. You don't want those resources deciding if you have quorum when there's a network glitch between the two data centers.