Adding null return for readStringUntil or equivalent for readBytesUntil

[Suisse00]

The documentation doesn't talk about what is returned when readStringUntil can't find the terminator, so I added it.

I added clarification that if read*Until don't find ther terminator all read characters are discarded.

Also I documented that if a terminator is found (but without previous data) the code manage it has if there is no terminator. (About that, I created https://github.com/arduino/ArduinoCore-avr/issues/491. We should be able to know about the difference)

[CLAassistant]

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