Add pybind11/compat/README.txt and a couple source code comments.

Author: rwgk

include/pybind11/compat/README.txt

@@ -0,0 +1,10 @@
+NOTE
+----
+
+The C++ code here
+
+** only depends on <Python.h> **
+
+and nothing else.
+
+DO NOT ADD CODE WITH OTHER EXTERNAL DEPENDENCIES TO THIS DIRECTORY.

include/pybind11/compat/pybind11_platform_abi_id.h

@@ -2,6 +2,9 @@
 
 // Copyright (c) 2024 The pybind Community.
 
+// To maximize portability:
+// DO NOT ADD CODE THAT REQUIRES C++ EXCEPTION HANDLING.
+
 #include "wrap_include_python_h.h"
 
 // Implementation detail. DO NOT USE ELSEWHERE.

include/pybind11/compat/wrap_include_python_h.h

@@ -12,6 +12,9 @@
 //   the standard headers on some systems, you must include Python.h before
 //   any standard headers are included.
 
+// To maximize portability:
+// DO NOT ADD CODE THAT REQUIRES C++ EXCEPTION HANDLING.
+
 // Disable linking to pythonX_d.lib on Windows in debug mode.
 #if defined(_MSC_VER) && defined(_DEBUG) && !defined(Py_DEBUG)
 // Workaround for a VS 2022 issue.