Mention or explain binding modes when describing patterns

• I have checked the latest main branch to see if this has already been fixed
• I have searched existing issues and pull requests for duplicates

URL to the section(s) of the book with this problem:
https://doc.rust-lang.org/nightly/book/ch06-00-enums.html
https://doc.rust-lang.org/nightly/book/ch18-00-patterns.html
https://doc.rust-lang.org/stable/book/ch15-06-reference-cycles.html#creating-a-reference-cycle

Description of the problem:
There's no mention in chapters 6 or 18 of how matching on a reference works when there's no reference operator in the pattern (i.e. binding modes / match ergonomics). In chapter 15, there is an example that implicitly uses this behaviour, but it is not explained or given a name, meaning people who want to learn more can't easily research the topic.

Suggested fix:
Add a page or section in chapter 18, either describing the behaviour, or naming it and pointing to the rust reference page on binding modes.

Start with the chapter 18 patterns section and compare it with the chapter 6 enums material and the chapter 15 reference-cycle example linked in the issue. Add a section or page that explains or names binding modes/match ergonomics and links to the Rust Reference; done means readers can identify and research the behavior.