Align documentation of storage class and pvcs on retroactive storage class assignment

The documentation on [default StorageClass](https://kubernetes.io/docs/concepts/storage/storage-classes/#default-storageclass) and [Retroactive default StorageClass assignment](https://kubernetes.io/docs/concepts/storage/persistent-volumes/#retroactive-default-storageclass-assignment) seem to have some identical content. I made a pull request #56587 as a proposal to align the content of these two topics on that subject.
And I would like to get your feedback whether these changes are an improvement or if the redundant content actually is better suited. Thanks for having a look at this 🙂

Review the default StorageClass and Retroactive default StorageClass assignment documentation linked in the issue, then compare the proposed changes in pull request #56587. Done means determining whether the duplicated content should be aligned or retained and recording feedback for the maintainers.

Written by the indexing model from the issue text.