Clarify/add `must_use` message for Rc/Arc/Weak::into_raw.
The current `#[must_use]` messages for `{sync,rc}::Weak::into_raw` ("`self` will be dropped if the result is not used") are misleading, as `self` is consumed and will *not* be dropped.
This PR changes their `#[must_use]` message to the same as `Arc::into_raw`'s[ current `#[must_use]` message](d573564575/library/alloc/src/sync.rs (L1482)) ("losing the pointer will leak memory"), and also adds it to `Rc::into_raw`, which is not currently `#[must_use]`.
72651306b3