Add docs

Author: encukou

Doc/c-api/intro.rst

@@ -30,6 +30,16 @@ familiar with writing an extension before  attempting to embed Python in a real
 application.
 
 
+Language version compatibility
+==============================
+
+Python's C API is compatible with C11 and C++11 versions of C and C++.
+
+This is a lower limit: the C API does not require features from later
+C/C++ versions.
+You do *not* need to enable your compiler's "c11 mode".
+
+
 Coding standards
 ================
 

Lib/test/test_cext/__init__.py

@@ -38,6 +38,9 @@ def test_build_c11(self):
 
     @unittest.skipIf(support.MS_WINDOWS, "MSVC doesn't support /std:c99")
     def test_build_c99(self):
+        # In public docs, we say C API is compatible with C11. However,
+        # in practice we do maintain C99 compatibility in public headers.
+        # Please ask the C API WG before adding a new C11-only feature.
         self.check_build('_test_c99_cext', std='c99')
 
     @support.requires_gil_enabled('incompatible with Free Threading')

Lib/test/test_cppext/__init__.py

@@ -29,6 +29,9 @@ def test_build(self):
         self.check_build('_testcppext')
 
     def test_build_cpp03(self):
+        # In public docs, we say C API is compatible with C++11. However,
+        # in practice we do maintain C++03 compatibility in public headers.
+        # Please ask the C API WG before adding a new C++11-only feature.
         self.check_build('_testcpp03ext', std='c++03')
 
     @unittest.skipIf(support.MS_WINDOWS, "MSVC doesn't support /std:c++11")