Rollup merge of #139416 - mejrs:metavar, r=tgross35

unstable book; document `macro_metavar_expr_concat`

Rendered:

![image](https://github.com/user-attachments/assets/25c6d1fa-dac4-4c28-869f-e053b9384e59)
![image](https://github.com/user-attachments/assets/caa11e61-6321-4646-bf53-a1d31fca39f4)
![image](https://github.com/user-attachments/assets/52dc788f-1764-4a75-9fc3-e0fdefd30642)
![image](https://github.com/user-attachments/assets/2fd2ba06-b8e3-4341-83ec-0ea686efdde2)

cc `@c410-f3r`

Author: matthiaskrgr