Clarify expected behavior in LINQ async example

[Copilot]

The LINQ async example in the C# asynchronous programming documentation was causing confusion about whether the shown approach was correct or problematic. The example demonstrated the correct implementation using .ToArray() for immediate evaluation, but the accompanying warning text suggested there was an issue with the approach, leaving readers uncertain about best practices.

Changes Made

Before: The documentation showed a correct LINQ example but included confusing warning text that made it unclear whether this was the recommended approach.

After: The documentation now clearly:

• States that the shown LINQ example is correct and safe because it uses .ToArray()
• Explains why .ToArray() ensures immediate task execution and concurrent processing
• Provides a contrasting "problematic approach" example without .ToArray()
• Shows the "recommended approach" with clear DO/DON'T guidance
• Includes explicit guidance to always use .ToArray() or .ToList() with async LINQ

Example of the clarification:

// DON'T do this - tasks won't start until enumerated
var getUserTasks = userIds.Select(id => GetUserAsync(id)); // No .ToArray()!
return await Task.WhenAll(getUserTasks); // Tasks start here, not above

// DO this - tasks start immediately
var getUserTasks = userIds.Select(id => GetUserAsync(id)).ToArray();
return await Task.WhenAll(getUserTasks);

This change eliminates the confusion identified in the issue by making it crystal clear that the LINQ approach with .ToArray() is the recommended way to create concurrent async tasks.

Fixes #27720.

Warning

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• learn.microsoft.com
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Internal previews

📄 File	🔗 Preview link
docs/csharp/asynchronous-programming/async-scenarios.md	docs/csharp/asynchronous-programming/async-scenarios

[BillWagner]

@copilot Please fix the three markdown lint issues.

[Copilot]

@copilot Please fix the three markdown lint issues.

Fixed the three markdown lint issues in commit 7cd9d78:

1. Removed trailing space from line 133
2. Added blank lines before fenced code blocks on lines 138 and 145

The linter now passes with 0 errors.

[BillWagner]

This LGTM and is ready for final review.