Mention the temporary scope of while let

I'm not an expert here, but because `while let` is a loop, it *has to*
drop temporaries when it loops, so it must have at least one temporary
scope, and empirically, the same scope includes the scrutinee and the
body because the body can use temporaries from the scrutinee.

Author: kpreid

src/destructors.md

@@ -208,6 +208,7 @@ smallest scope that contains the expression and is one of the following:
 * The body expression for a match arm.
 * Each operand of a [lazy boolean expression].
 * The pattern-matching condition and consequent body of [`if let`] ([destructors.scope.temporary.edition2024]).
+* The pattern-matching condition and loop body of [`while let`].
 * The entirety of the tail expression of a block ([destructors.scope.temporary.edition2024]).
 
 > [!NOTE]