Change slave to device_id and slave= to device_id=. (#2600)

Author: janiversen

API_changes.rst

@@ -2,13 +2,15 @@ API changes
 ===========
 Versions (X.Y.Z) where Z > 0 e.g. 3.0.1 do NOT have API changes!
 
------------------
-API changes 3.9.0
+API changes 4.0.0
 -----------------
 - Python 3.9 is reaching end of life, and no longer supported.
   Depending on the usage the code might still work
-- Start*Server, custom_functions -> custom_pdu (handled by Modbus<x>Server)
-- payload removed (replaced by "convert_combined_to/from_registers")
+- Start*Server, custom_functions is now custom_pdu (handled by Modbus<x>Server)
+- ModbusSlaveContext replaced by ModbusDeviceContext
+- payload removed (replaced by "convert_to/from_registers")
+- slave=, slaves= replaced by device_id=, device_ids=
+- slave request names changed to device
 
 API changes 3.8.0
 -----------------

doc/index.rst

@@ -8,6 +8,7 @@ Please select a topic in the left hand column.
    :caption: Contents:
    :hidden:
 
+   source/upgrade_40
    source/api_changes
    source/client
    source/server