Update analogWrite doc

[stefankaters]

You need to call pinMode() before calling analogWrite();

[kengdahl]

If nobody objects I will merge this.

[per1234]

@katerss1 @kengdahl which board did you find this was necessary for?

I know that, for Arduino AVR Boards, the previous documentation was correct:
https://github.com/arduino/ArduinoCore-avr/blob/1.8.3/cores/arduino/wiring_analog.c#L108-L111

	// writing with them.  Also, make sure the pin is in output mode
	// for consistenty with Wiring, which doesn't require a pinMode
	// call for the analog output pins.
	pinMode(pin, OUTPUT);

I haven't looked at other official boards platforms though.

[kengdahl]

@per1234
Without the pinMode(); it is not working properly on the MKR boards, i.e. SAMD architecture. So there is an inconsistency between our boards.

[kengdahl]

@per1234 did some research and:
Is it necessary to call pinMode(pin, OUTPUT) when using analogWrite()?

• ARC32 (Arduino 101): no
• AVR (Uno): no
• Mbed (Nano 33 BLE): no
• Mbed (Portenta H7 (M7 core)): no
• Mbed (Portenta H7 (M4 core)): no
• megaAVR (Nano Every): no
• SAM (Due): no
• SAMD (MKR WiFi 1010): no

The test sketch:

void setup() {
  analogWrite(3, 125);
}
void loop() {}

So all the official Arduino boards platforms behave consistently in regards to not needing to set the pin mode to OUTPUT when using analogWrite() and the documentation is fine as it is now in this regard.