docs: Remove radio.RATE_250KBIT as it has been dropped from V2. (bbcmicrobit#731)

The constant is still present in the micro:bit V1 MicroPython
port, but it is not in the V2 MicroPython port.
The value from the constant (`2`) can be used as an argument in
the V2 MicroPython `radio.config(data_rate=2)`, but because it is
not officially supported in the V2 nRF52833 microcontroller we
cannot guarantee it will always work, so the constant is
deprecated in V1 and removed in V2.

Author: microbit-carlos

docs/radio.rst

@@ -28,17 +28,19 @@ We assume you have done this for the examples below.
 Constants
 =========
 
-.. py:attribute:: RATE_250KBIT
-
-    Constant used to indicate a throughput of 256 Kbit a second.
-
 .. py:attribute:: RATE_1MBIT
 
-    Constant used to indicate a throughput of 1 MBit a second.
+    Constant used to indicate a throughput of 1 Mbit a second.
 
 .. py:attribute:: RATE_2MBIT
 
-    Constant used to indicate a throughput of 2 MBit a second.
+    Constant used to indicate a throughput of 2 Mbit a second.
+
+.. py:attribute:: RATE_250KBIT
+
+    **Deprecated**.
+    This rate is possible with micro:bit V1, but it is not guaranteed to work
+    on V2, so it has been deprecated for compatibility reasons.
 
 
 Functions
@@ -90,8 +92,7 @@ Functions
 
     The ``data_rate`` (default=radio.RATE_1MBIT) indicates the speed at which
     data throughput takes place. Can be one of the following contants defined
-    in the ``radio`` module : ``RATE_250KBIT``, ``RATE_1MBIT`` or
-    ``RATE_2MBIT``.
+    in the ``radio`` module : ``RATE_1MBIT`` or ``RATE_2MBIT``.
 
     If ``config`` is not called then the defaults described above are assumed.