add documentation for hvdc lines (#805)

---------

Signed-off-by: Etienne LESOT <etienne.lesot@rte-france.com>

pypowsybl/network/impl/network.py

@@ -1776,8 +1776,8 @@ def get_hvdc_lines(self, all_attributes: bool = False, attributes: List[str] = N
         Notes:
             The resulting dataframe, depending on the parameters, will include the following columns:
 
-              - **converters_mode**:
-              - **target_p**: (in MW)
+              - **converters_mode**: the mode of the converter stations. It can be either SIDE_1_RECTIFIER_SIDE_2_INVERTER or SIDE_1_INVERTER_SIDE_2_RECTIFIER
+              - **target_p**: active power target (in MW)
               - **max_p**: the maximum of active power that can pass through the hvdc line (in MW)
               - **nominal_v**: nominal voltage (in kV)
               - **r**: the resistance of the hvdc line (in Ohm)