[doc] update doc

Author: vimior

doc/api/xarm_api.md

@@ -732,6 +732,7 @@ xArm-Python-SDK API Documentation (V1.16.0): class XArmAPI in module xarm.wrappe
 >     2. the Six-axis Force Torque Sensor is required (the third party is not currently supported)  
 >   
 > :param is_raw: get the raw data or not.  
+>     1. only available if firmware_version >= 2.6.109  
 > :return: tuple((code, exe_ft))  
 >     code: See the [API Code Documentation](./xarm_api_code.md#api-code) for details.  
 >     ft_data: only when code is 0, the returned result is correct.  

xarm/wrapper/xarm_api.py

@@ -3365,6 +3365,7 @@ def get_ft_sensor_data(self, is_raw=False):
             2. the Six-axis Force Torque Sensor is required (the third party is not currently supported)
 
         :param is_raw: get the raw data or not.
+            1. only available if firmware_version >= 2.6.109
         :return: tuple((code, exe_ft))
             code: See the [API Code Documentation](./xarm_api_code.md#api-code) for details.
             ft_data: only when code is 0, the returned result is correct.