Document callback functionality with SMAC

[eddiebergman]

Thanks to @thomashlvt, we have new functionality in SMAC to utilize callbacks which can be done through autosklearn. We would require documenting how this might be used and create an example, perhaps even changing the API names slightly to make it more intuitive.

Relevant issues:

• [Feature] Hook to implement early stopping #1223
• SMAC Interrupt optimization through IncorporateRunResultCallback SMAC3#765

[mfeurer]

Closed via #1476.