Update performance.rst

[D-Bullock]

Seemingly there is no longer IDataPoint to extend. :(
Found as I was trying to implement the suggestions

[VisualMelon]

There is IDataPointProvider, but I think this change is helpful in any case.

[D-Bullock]

As it turns out you can't extend data point - it's a struct. So it really must suggest extending IDataPointProvider

[VisualMelon]

Good point on the working. Feel free to force-push a change to that effect in this branch. I don't think we have any particular contribution requirements, so I'll merge this after a couple of days to give other people a change to provide commentary.