Documenting missing unit test coverage. (#3673)

Author: rwgk

include/pybind11/pybind11.h

@@ -991,6 +991,7 @@ class cpp_function : public function {
 #if PY_VERSION_HEX >= 0x03030000
             // Attach additional error info to the exception if supported
             if (PyErr_Occurred()) {
+                // #HelpAppreciated: unit test coverage for this branch.
                 raise_from(PyExc_TypeError, msg.c_str());
                 return nullptr;
             }