[DOC] `rapids_cython_create_modules()` should document the fact that it reads `CYTHON_FLAGS`

[Jacobfaib]

rapids_cython_create_modules() inspects the variable CYTHON_FLAGS. It's not obvious that this specific variable is consulted.

This should probably be documented, or perhaps exposed as an additional argument to rapids_cython_create_modules(). I guess it is supposed to mimic the CMAKE_CXX_FLAGS and friends. But if that is the case, the user might expect to set e.g. CMAKE_CYTHON_FLAGS or CMAKE_CYTHON_FLAGS_DEBUG.

[vyasr]

I agree, this would be good to do. If you have a few moments we'd love a contribution! I originally chose CYTHON_FLAGS to match the behavior of the legacy scikit-build's Cython CMake layer, but I'd welcome improvements to that design now that we're pretty much rolling our own Cython invocations. I do hope to eventually switch over to using cython-cmake (and longer-term a native Cython solution) so I'd also be happy to propose any such improvements upstream.