[doc] Simplify the upsert resolution Wiki page to agree with the actual implementation

Now that #283 has landed, the documentation at https://github.com/mozilla/mentat/wiki/Transacting:-upsert-resolution-algorithm is stale. Luckily, it's stale 'cuz the implementation is simpler than the algorithm description! This ticket is a marker for us to loop back to the Wiki and make sure what we actually do in the Rust implementation is accurately described in the Wiki.

See also #207, which might be addressed concurrently.