[3.7] Document language feature inference-using-bounds

[MaryaBelanger]

Design doc here: https://github.com/dart-lang/language/blob/main/accepted/future-releases/3009-inference-using-bounds/design-document.md

To do: evaluate

[eernstg]

In the documentation of the inference-using-bounds feature it definitely seems useful to have a reference to @chloestefantsova's document.

It gets more complex when it comes to the general documentation of inference. I don't really have a complete idea about what could be done, but it seems likely to me that the graphs could be used to illustrate the treatment of a non-trivial case (the treatment after the change), and the general reader could benefit from that (with some text which could be very similar to the text in the document, I think). The "before" variant could be omitted here, because it's intended to describe how inference works (not how it was changed in order to add the inference-using-bounds feature). Does that make sense?