export_native_api.md: Relax the "ground rule" (#1577)

Nothing wrong with passing structures and function pointers
as far as the native code is aware of and careful about the
ABI used in the wasm module.

doc/export_native_api.md

@@ -189,7 +189,7 @@ void foo2(wasm_exec_env_t exec_env,
 
 The runtime builder should ensure not broking the memory sandbox when exporting the native function to WASM. 
 
-A few key ground rules:
+A few recommendations:
 
 - Never pass any structure/class object pointer to native (do data serialization instead)
 - Do the pointer address conversion in the native API if "$\*" is not used for the pointer in the function signature