whitepaper: Consider explaining probing more (perhaps it's another signal?)

[bwplotka]

Solid suggestion from our final 1.0 community review period by @beorn7

Which, as a side thought, reminds me of the question if "probing" should be an O11y signal as well. It isn't really part of the metrics signal. It is certainly mostly about the "what" than the "why", so you could say it's not part of O11y because, in the modern world, you usually don't do it to infer internal state from the outside. But if we use O11y in the widest possible sense (i.e. synonymous to "modern monitoring"), then acknowledging the still existing usefulness of probing might be useful