Optimistic locking in the documentation

[shayantabrizi]

I did not find a section for optimistic locking in the documentation for the new version. There was one for the previous version here. It may be good to explain it in more detail in the latest documentation.

[meistermeier]

You are right, this is truly missing.

[meistermeier]

Update documentation in 6.0 (7cb4da9), 6.1 (1d23beb) and main (50e3d66).