Expand all excerpts had a doc comment describing it as expanding all
excerpts, but in practice it only expanded the excerpt that was the most
relevant.

I fixed that to make it expand all excerpts. 

video:


https://github.com/user-attachments/assets/9858ebda-199c-4f72-8a2f-3cd606b0eff4

Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Closes #54651 

Release Notes:

- editor: `expand excerpts` now has correct documentation explaining its
function.