docs: smart_sort should be documented

[injust]

The smart_sort config option is missing some documentation. I had to refer to #1885 to understand what it actually does.

It should probably be documented in https://docs.atuin.sh/configuration/config/ as well as config.toml.

[ellie]

Thanks for the issue! I've actually left it as undocumented for a while now intentionally, as while it seemed to work I wanted to test it some more before advertising it and committing to supporting it. It's probably fine now though