Clarify that pytest-benchmark's pedantic API is unsupported

[seddonym]

The codspeed docs currently say:

Creating benchmarks with pytest-codspeed is the same as with the pytest-benchmark API. So if you already have benchmarks written with it, you can start using CodSpeed right away 🚀

This isn't true, though: pedantic mode is not supported. It would be helpful if the docs could clarify what subset of the pytest-benchmark API is supported (together with, perhaps, a version number for pytest-benchmark?)