Document interaction with systemd-homed --storage=fscrypt

[josephlr]

systemd-homed allows for fscrypt-encrypted home directories.

However, this does not use the fscrypt userspace tool, just the kernel functionality. We should document the tradeoffs of using homectl vs this tool. We should also make it clear that this userspace tool doesn't do anything to fscrypt encrypted home directories. We could (in theory) also add checks in the command-line tool to properly handle cases where a user tries to operate on the systemd-managed home directories.