Fix datetime benchmark uploading and document

[sffc]

Since about 2024-11-15, the datetime benchmarks have not been uploading successfully.

The benchmark CI has failed in a similar way before, and @robertbastian has always fixed it. We should document how to go about fixing it so as to not have a single point of failure.

[Manishearth]

This is no longer a 2.0 blocker (the benchmark fixed itself), but we should still document things.

[robertbastian]

This is not an upload error. The benchmark failed (even though the "Run benchmark" step doesn't report as failing), so there's nothing to upload. I have previously fixed it by fixing the benchmark code, which for some reason is not tested in CI.