Add a note about filesystem overhead for PVC size request

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

Signed-off-by: Alex Kalenyuk <[email protected]>

Author: akalenyu

content/en/docs/concepts/storage/persistent-volumes.md

@@ -830,6 +830,11 @@ the request is for storage. The same
 [resource model](https://git.k8s.io/design-proposals-archive/scheduling/resources.md)
 applies to both volumes and claims.
 
+{{< note >}}
+For `Filesystem` volumes, the storage request refers to the "outer" volume size (i.e. the allocated size from the storage backend).
+This means that the writeable size may be slightly lower for providers that build a filesystem on top of a block device, due to filesystem overhead. This is especially visible with XFS, where many metadata features are enabled by default.
+{{< /note >}}
+
 ### Selector
 
 Claims can specify a