doc: Clarify the lifetime returned by `Box::leak`

`Box::leak` mentions that it can return a `'static` reference, but it
wasn't immediately clear to me why it doesn't always do so.  This is
because of the `T: 'a` constraint needed to form a valid reference, and
in general we want to be more flexible than requiring `T: 'static`.
This patch tries to clarify the relationship between `T` and `'a`.

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