Custom Terrain Tool - Where to start?

I just tried the custom terrain tool samples that are available on github and they are awesome. To be precise, these here:

https://github.com/Unity-Technologies/TerrainToolSamples

Is there a documentation about how to create a custom terrain tool and where to start? I don't mind using the samples as a reference, the code is very well written and easy to understand, but if there's a documentation with even more information that's not convered in the samples, it would be nice if someone could point me to it. Thank you!