Determine whether a relay parent digest item should go in the parachain block header

[JoshOrndorff]

FRAME's pallet parachain system includes a log on each parachain block header indicating the relay parent's storage root. https://github.com/paritytech/polkadot-sdk/blob/408af9b32d95acbbac5e18bee66fd1b74230a699/cumulus/pallets/parachain-system/src/lib.rs#L577-L585

I'm not clear when, if ever this is used, and the Tuxedo parachain seems to be working fine without it currently. This issue is to determine exactly why that log is included and when, if ever, it is used, then to determine whether Tuxedo needs it, and if so, to include it in the code.