test: document the xdist_group values

Author: nedbat

pyproject.toml

@@ -117,6 +117,12 @@ markers = [
     "expensive: too slow to run during \"make smoke\"",
 ]
 
+### pytest.mark.xdist_group() values, and why:
+# "virtualenv_test": because of an expensive session-scoped fixture
+# "compare_test": Because of shared-file manipulations (~/tests/actual/testing)
+# "get_zip_bytes_test": no idea why this one fails if run on separate workers
+# "needs_pth": tests that create .pth files in shared locations
+
 # How come these warnings are suppressed successfully here, but not in conftest.py??
 filterwarnings = [
     # Sample 'ignore':