Document temporary scoping for destructuring assignments #1992
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Pursuant to rust-lang/rust#145838 (comment), I've added a note and some examples detailing how destructuring assignment expressions effectively introduce two scopes for its temporaries, whereas basic assignment expressions don't introduce any.
I've included it alongside the description of destructuring assignment desugaring, but I could see it being at home in the destructors chapter as well. I also considered adding the assigned value operand of a destructuring assignment operand to the list of temporary scopes, but it's a bit more complicated than that since it has two scopes, so I figured it would be simplest to talk directly in terms of the desugaring, where the two scopes are visible.
I've used borrowck errors to demonstrate scoping in the tests since the test scaffolding is much simpler in that case than for testing runtime drop order, but a runtime drop order comparison may help further illustrate it.