need to document the process for expanding existing filesystems on expanded volumes

When a user expands a volume that has an existing filesystem, that filesystem will need to be expanded from within a VM. Depending on the OS image being used on a given instance, this may happen automatically via various cloud utils or might need to be done manually by the user. In the case of manual steps this would be using `resize2fs` for ext-based filesystems, `xfs_growfs` for xfs, etc.