Explain the IntelliSense/C++20-module preprocessor block in Base code

en/03_Drawing_a_triangle/00_Setup/00_Base_code.adoc

@@ -38,6 +38,22 @@ import vulkan_hpp;
 #include <cstdlib>
 
 class HelloTriangleApplication {
+----
+
+The `#if defined(__INTELLISENSE__) || !defined(USE_CPP20_MODULES)` block picks
+between two ways of pulling in the Vulkan-Hpp API, both of which provide the
+same functions, structures, and enumerations. By default we include the
+traditional `<vulkan/vulkan_raii.hpp>` header. If you enable C{pp}20 modules
+(`-DENABLE_CPP20_MODULE=ON`), the `USE_CPP20_MODULES` define set by CMake
+makes the code `import vulkan_hpp;` instead. Either way, Visual Studio and VS
+Code's IntelliSense engine doesn't yet understand `import` for modules and
+will flag it as an error even though the code compiles fine, so we fall back
+to the header whenever `__INTELLISENSE__` is defined (which IntelliSense
+itself defines while analyzing the code) or when modules are turned off
+entirely.
+
+[,c++]
+----
 public:
     void run() {
         initVulkan();
@@ -76,12 +92,9 @@ int main()
 }
 ----
 
-We first include the Vulkan-Hpp RAII header by default, which provides the
-functions, structures and enumerations. If you enable C{pp}20 modules
-(`-DENABLE_CPP20_MODULE=ON`), the code will `import vulkan_hpp;` instead via the
-`USE_CPP20_MODULES` define set by CMake. The `stdexcept` and `iostream` headers
-are included for reporting and propagating errors. The `cstdlib` header
-provides the `EXIT_SUCCESS` and `EXIT_FAILURE` macros.
+The `stdexcept` and `iostream` headers are included for reporting and
+propagating errors. The `cstdlib` header provides the `EXIT_SUCCESS` and
+`EXIT_FAILURE` macros.
 
 The program itself is wrapped into a class where we'll store the Vulkan objects
 as private class members and add functions to initiate each of them, which will