doc: update design

assets/c-friendly-ffi-design/c-cpp-headers.md

@@ -26,6 +26,19 @@ CError WFStartup(void);
 #endif  // __cplusplus
 ```
 
+## Parameter Declarations
+
+A declaration of an exported FFI function in the C header file generally looks like
+the following:
+
+```c++
+CError FBAdd(
+    OMRF_IN_PARAM_TY(uint32_t, in_a),
+    OMRF_IN_PARAM_TY(uint32_t, in_b),
+    OMRF_OUT_PARAM_TY(uint32_t, out_rv),
+    OMRF_OUT_PARAM_TY(bool, out_overflow));
+```
+
 ## Required Header Files
 
 The generated C/C++ header files must include the appropriate header files for the