Remove search_on_input from docs (#52435)

## Context

<img width="1610" height="394" alt="CleanShot 2026-03-25 at 15 32 24@2x"
src="https://github.com/user-attachments/assets/704753d5-17c0-4857-8599-718b310211c8"
/>


Although search_on_input is a merged feature, it doesn't look like it's
live yet. As a result, some people might be trying to activate the
feature without success.

This commit removes its reference from the docs.

## How to Review

- Check if the feature is live before merging

## Self-Review Checklist

<!-- Check before requesting review: -->
- [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

Release Notes:

- N/A

docs/src/reference/all-settings.md

@@ -3462,12 +3462,6 @@ Non-negative `integer` values
 - Setting: `regex`
 - Default: `false`
 
-### Search On Input
-
-- Description: Whether to search on input in project search.
-- Setting: `search_on_input`
-- Default: `true`
-
 ### Center On Match
 
 - Description: Whether to center the cursor on each search match when navigating.