The developer application program interface (API) is documented using the program Doxygen from the source code itself. It is updated for every major and minor version release.
Bug fix releases (e.g., 2.0.0, vs. 2.0.1, vs. 2.0.2) do not change API at all, although many times the documentation will be updated to correct errors or add additional documentation.
Minor versions (e.g., 2.0 vs. 2.1) will add function calls, but will be otherwise backwards-compatible. Again, documentation may be updated to correct errors in the text or add additional documentation. Such changes should be considered to apply to older versions as well.