doc: add the doc for c++ binding according to c binding

wfassoc-cdylib/cbindgen/wfassoc.h

@@ -22,6 +22,12 @@
 
 
 #ifdef __cplusplus
+/**
+ * @brief Windows File Association C/C++ API namespace
+ *
+ * This namespace only provide the raw binding to C-compatible wfassoc API in C++.
+ * For the C++ RAII wrappers of this, please see @ref wfassocpp namespace.
+ */
 namespace wfassoc {
 #endif // __cplusplus