Update NFS Docs to include File Locking client flag

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Summary

Followed the guide for using NFS storage (https://docs.gitlab.com/ee/administration/high_availability/nfs.html) to set up mounts for our Omnibus container within Kubernetes. This resulted in the instance being responsive for ~15 minutes before hanging. Other symptoms included not being able to acquire file locks on certain log files, and running gitlab-ctl restart would hang/timeout on sidekiq.

Possible fixes

Adding local_lock=flock to the mount options fixed it. Consider adding this to the NFS documentation as a potential problem as well as adding the flag in the example client options.

Thanks!