[skip ci] Explain: conduit feature only covers from-Python-to-C++ conversions (#5740)

Author: rwgk

include/pybind11/conduit/pybind11_conduit_v1.h

@@ -6,6 +6,11 @@
 
 * including pybind11 versions with different PYBIND11_INTERNALS_VERSION's.
 
+    * NOTE: The conduit feature
+            only covers    from-Python-to-C++ conversions, it
+            does not cover from-C++-to-Python conversions.
+            (For the latter, a different feature would have to be added.)
+
 The naming of the feature is a bit misleading:
 
 * The feature is in no way tied to pybind11 internals.

tests/test_cpp_conduit.py

@@ -151,8 +151,8 @@ def test_exo_planet_c_api_premium_traveler(premium_traveler_type):
 
 def test_home_planet_wrap_very_lonely_traveler():
     # This does not exercise the cpp_conduit feature, but is here to
-    # demonstrate that the cpp_conduit feature does not solve all
-    # cross-extension interoperability issues.
+    # demonstrate that the cpp_conduit feature does not solve
+    # cross-extension base-and-derived class interoperability issues.
     # Here is the proof that the following works for extensions with
     # matching `PYBIND11_INTERNALS_ID`s:
     #     test_cpp_conduit.cpp: