Add a note about filesystem overhead for PVC size request

[akalenyu]

Description

Though this is not explicitly stated, it seems acceptable that
once a provider builds a filesystem on top of a block device,
some writeable capacity is lost:
container-storage-interface/spec#338

Issue

Closes: #

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[akalenyu]

/cc @jsafrane @nixpanic

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[Ritikaa96]

for triage and reviews:
/sig storage

[nixpanic]

Thanks!

/lgtm
/assign saad-ali

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: 2baf53bb97e7c9b049ee4ceb206cf38850fa0409