update ARM section about extended-info4/6-upgrade commands

Looks like this feature is working, but documentation needs clarification WHEN exactly commands extended-info4-upgrade and extended-info6-upgrade are needed.

Looks like extended-info4-upgrade is needed when Kea is upgraded from 2.2.1 to current (but not from 2.4.1 to current) and extended-info6-upgrade is needed when upgrading from 2.2.1 AND from 2.4.1 to current master.

Please also mention that if once used, this command won't be needed in future upgrades

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information