Because every feature of SymPy must have a test case, when you are not sure how to use something, just look into the tests/ directories, find that feature and read the tests for it, that will tell you everything you need to know.
Most of the things are already documented though in this document, that is automatically generated using SymPy’s docstrings.
Click the “modules” (Module Index) link in the top right corner to easily access any SymPy module, or use this contens:
All contributions are welcome. If you’d like to improve something, look into the sources if they contain the information you need (if not, please fix them), otherwise the documentation generation needs to be improved (look in the doc/ directory).