Auto merge of #46168 - durka:macro-backtrace, r=durka

mention nightly in -Z external-macro-backtrace note

Fix #46167 by mentioning that you need nightly in the message that tells you to pass `-Z external-macro-backtrace`.

Rationale:

1. The reason for having this message is to increase discoverability of the functionality. If the message is only shown on nightly it's less disoverable.
2. The same approach is taken if you call a const fn in const context without its feature gate (previously, if you called it without `#![feature(const_fn)]`).

cc @kennytm

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