Document limitation of Serial1.begin() config value for Nano 33 BLE boards

Add a warning that use of any Serial1.begin() config value other than SERIAL_8N1 on the Arduino Nano 33
BLE boards will cause an Mbed OS crash.

Author: per1234

Language/Functions/Communication/Serial/begin.adoc

@@ -113,6 +113,8 @@ Thanks to Jeff Gray for the mega example
 [float]
 === Notes and Warnings
 For USB CDC serial ports (e.g. `Serial` on the Leonardo), `Serial.begin()` is irrelevant. You can use any baud rate and configuration for serial communication with these ports. See the list of available serial ports for each board on the link:../../serial[Serial main page].
+
+The only `config` value supported for `Serial1` on the Arduino Nano 33 BLE and Nano 33 BLE Sense boards is `SERIAL_8N1`.
 [%hardbreaks]
 
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