Specify behavior of analogWrite on Nano 33 BLE/BLE Sense

[alixinne]

After working with the Arduino Nano 33 BLE, I discovered that calling analogWrite on more than 4 pins will lock up the board. This is confirmed by https://forum.arduino.cc/t/arduino-nano-33-ble-damaged/639669/2.

I think the reference documentation should include this quirk since it's currently not documented anywhere, and might be confusing for Nano 33 BLE/BLE Sense board users.

I did replace footnote asterisks with superscript one, two and three since otherwise it would understand *** as bold formatting. I don't know if there is another way this could be done.

[CLAassistant]

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