Beyond an overview in the manual. I’m not aware of any documentation outside of the source in outline.el which defines outline-regexp to match the beginning of a heading, outline-heading-end-regexp to match the end of a heading, outline-search-function to advance to the next heading, and outline-level to figure out the level of the heading which can be used with outline-heading-alist.

python.el provides values for outline-regexp, outline-heading-end-regexp, and outline-level which mostly work for me except for decorators and parametrized test cases.

The Stack Exchange discussion gives an example of customizing the values using a hook into python-mode.