Put your question regarding the usage of snappy, the snap-python interface or anything else related to Python and SNAP into this category. This could be questions regarding the access of the SNAP API from Python or implementing an operator in Python, all questions like this are best suited here.
Question regarding Sen2Cor should go into the sen2cor category.
General documentation about snappy can be found in How to use the SNAP API from Python of the Developer Guide. There is also a page which explains how python can be configured to use it with snappy.
Beside the mentioned pages you can also look at the examples in the snap-python module and in the examples project.
Writing your own operator in Python is explained at How to write a processor in Python (still in progress). It is also good to know the SNAP Java API documentation.