docs: Explain what scrollbar marks represent (#42130)

## Summary

Adds explanations for what each type of scrollbar indicator visually
represents in the editor.

## Description

This PR addresses the issue where users didn't understand what the
colored marks on the scrollbar mean. The existing documentation
explained how to toggle each type of mark on/off, but didn't explain
what they actually represent.

This adds a brief, clear explanation after each scrollbar indicator
setting describing what that indicator shows (e.g., "Git diff indicators
appear as colored marks showing lines that have been added, modified, or
deleted compared to the git HEAD").

## Fixes

Closes #31794

## Test Plan

- Documentation follows the existing style and format of
`docs/src/configuring-zed.md`
- Each explanation is concise and immediately follows the setting
description
- Language is clear and user-friendly

Release Notes:

- N/A

docs/src/configuring-zed.md

@@ -929,6 +929,8 @@ List of `string` values
 - Setting: `cursors`
 - Default: `true`
 
+Cursor indicators appear as small marks on the scrollbar showing where other collaborators' cursors are positioned in the file.
+
 **Options**
 
 `boolean` values
@@ -939,6 +941,8 @@ List of `string` values
 - Setting: `git_diff`
 - Default: `true`
 
+Git diff indicators appear as colored marks showing lines that have been added, modified, or deleted compared to the git HEAD.
+
 **Options**
 
 `boolean` values
@@ -949,6 +953,8 @@ List of `string` values
 - Setting: `search_results`
 - Default: `true`
 
+Search result indicators appear as marks showing all locations in the file where your current search query matches.
+
 **Options**
 
 `boolean` values
@@ -959,6 +965,8 @@ List of `string` values
 - Setting: `selected_text`
 - Default: `true`
 
+Selected text indicators appear as marks showing all occurrences of the currently selected text throughout the file.
+
 **Options**
 
 `boolean` values
@@ -969,6 +977,8 @@ List of `string` values
 - Setting: `selected_symbol`
 - Default: `true`
 
+Selected symbol indicators appear as marks showing all occurrences of the currently selected symbol (like a function or variable name) throughout the file.
+
 **Options**
 
 `boolean` values
@@ -979,6 +989,8 @@ List of `string` values
 - Setting: `diagnostics`
 - Default: `all`
 
+Diagnostic indicators appear as colored marks showing errors, warnings, and other language server diagnostics at their corresponding line positions in the file.
+
 **Options**
 
 1. Show all diagnostics: