Document & extend testing for rotation conventions (#16200)

* Document Luanti rotation conventions
* Add test for setPitchYawRollRad (entity) rotation conventions
* Test and document that `vector.rotate` uses (extrinsic) Z-X-Y rotation order

src/script/lua_api/l_object.cpp

@@ -1158,6 +1158,8 @@ int ObjectRef::l_set_rotation(lua_State *L)
 
 	v3f rotation = check_v3f(L, 2) * core::RADTODEG;
 
+	// Note: These angles are inverted before being applied using setPitchYawRoll,
+	// hence we end up with a right-handed rotation
 	entitysao->setRotation(rotation);
 	return 0;
 }