Rollup merge of #134321 - dtolnay:docassocconst, r=fmease

Hide `= _` as associated constant value inside impl blocks

Closes #134320.

### Before:

<img src="https://github.com/user-attachments/assets/19d28811-45d2-4563-9726-f40c6af411c6" width="300">&nbsp;<img src="https://github.com/user-attachments/assets/1ecf8764-97ce-47f0-87fa-3b174d2fc578" width="300">

### After:

<img src="https://github.com/user-attachments/assets/6408c4ca-b1c4-42e4-884b-248833a4865f" width="300">&nbsp;<img src="https://github.com/user-attachments/assets/df2f6981-16f6-409f-8abb-73c0a4a71d6b" width="300">

r? `@fmease`

Author: jhpratt