improve documentation for `client.points.upsert` in readme

[jronallo]

The readme only shows the "batch" approach for uploading points. It is also valid, even with this client, to send points with the "points" key as a more "record-oriented equivalent" as the Qdrant documentation states. As the batch approach which splits out the data might be awkward for some, it would be nice to show the "points" record-oriented approach as well.

Additionally the comment for the upsert section should be better description rather than "Get a single data object."

https://qdrant.tech/documentation/concepts/points/#upload-points

[andreibondarev]

@jronallo I would really appreciate a PR if you're able to work on this! Thank you!