docs(LocalKey<T>): clarify that T's Drop shouldn't panic

[ShE3py]

Clarify that should a TLS destructor panics, the process will abort.

Also, an abort may be obfuscated as the process can be terminated with SIGSEGV or STATUS_STACK_BUFFER_OVERRUN (i.e., SIGABRT is not guaranteed), so explicitly prints that the process was aborted.

Context:
https://users.rust-lang.org/t/status-stack-buffer-overrun-on-windows-without-any-usage-of-unsafe/128417

@rustbot label -T-compiler

[rustbot]

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[rustbot]

The Miri subtree was changed

cc @rust-lang/miri