Rollup merge of #147470 - ia0:immediate-abort, r=Mark-Simulacrum

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 rust-lang/rust#146974 for additional context.

See rust-lang/rust#147286 and rust-lang/cargo#16042 for the revelant tracking issues.

Author: Zalathar