Auto merge of #41476 - abonander:book_proc_macro, r=nrc

Document the `proc_macro` feature in the Unstable Book

Discusses the `proc_macro` feature flag and the features it enables:

* Implicit enable of `extern_use_macros` feature and how to import proc macros
* Error handling in proc macros (using panic messages)
* Function-like proc macros using `#[proc_macro]` and a usage example for creating and invoking
* Attribute-like proc macros using `#[proc_macro_attribute]` and a usage example for creating and invoking

[Rendered](https://github.com/abonander/rust/blob/book_proc_macro/src/doc/unstable-book/src/language-features/proc-macro.md)

0 files changed, 0 insertions, 0 deletions