Clarify setup.py behaviour

Author: hejamu

setup.py

@@ -89,7 +89,17 @@ def detect_openmp():
     mathlib = [] if os.name == "nt" else ["m"]
 
     has_openmp = detect_openmp()
-    # If we are building a source distribution, the c code is already generated
+
+
+    # There are two cases to consider when running setup.py:
+    # 1. We are building from source, so we need cython to generate the .c files
+    #    from the .pyx files.
+    # 2. we are building from a source distribution (sdist), in which case the
+    #    .c files are included and we don't need Cython.
+    # If we are installing from a wheel, the binary is already built and the
+    # setup.py is not run at all.
+    # We can detect a sdist by checking for the presence of a setup.cfg file,
+    # which is generated by setuptools when building the sdist.
     use_cython = not Path("./setup.cfg").exists()
     source_suffix = ".pyx" if use_cython else ".c"