Update powermax_nfs.yaml

[rawstorage]

updated NFS storage Class with additional commends an changed name of StorageClass from FC to NFS

Description

Making requirements clear in sample storage class.

GitHub Issues

List the GitHub issues impacted by this PR:
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Checklist:

• [ x] Have you run format,vet & lint checks against your submission?
• Have you made sure that the code compiles?
• Did you run the unit & integration tests successfully?
• Have you maintained at least 90% code coverage?
• Have you commented your code, particularly in hard-to-understand areas
• Have you done corresponding changes to the documentation
• Did you run tests in a real Kubernetes cluster?
• [x ] Backward compatibility is not broken

How Has This Been Tested?

Documentation change only