Clarify how to remediate the panic_immediate_abort error
Users who build `core` for the sole purpose of enabling `panic_immediate_abort` might expect "`panic_immediate_abort` is now a real panic strategy" to mean that setting `panic = "immediate-abort"` in `Cargo.toml` or `-Cpanic=immediate-abort` in `RUSTFLAGS` to be sufficient for migration. But this is not the case, `core` still needs to be built for those changes to take effect.
See https://github.com/rust-lang/rust/issues/146974 for additional context.
See https://github.com/rust-lang/rust/issues/147286 and https://github.com/rust-lang/cargo/issues/16042 for the revelant tracking issues.
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