Add explanation of why the when contextual keyword is better than if/else in catch blocks

[Copilot]

This PR addresses the lack of explanation for why C#'s when contextual keyword provides advantages over using if/else statements inside catch blocks. The key technical difference is that exception filters don't unwind the call stack, while traditional catch blocks do.

Changes Made

Enhanced exception-handling-statements.md

Added a comprehensive "Exception filters vs. traditional exception handling" section that explains:

• Exception filters (when) evaluate before stack unwinding, preserving the original call stack and local variables
• Traditional catch blocks unwind the stack before entering the handler, potentially losing debugging information
• Performance benefits when no filter matches (no unwinding overhead)
• Cleaner code organization for multiple conditions on the same exception type

Updated when.md

Added context explaining the advantages of exception filters with a cross-reference to the detailed explanation in the exception handling statements documentation.

New Code Examples

Created comprehensive working examples in WhenFilterExamples.cs demonstrating:

• Direct comparison between exception filters and if/else approaches
• Multiple condition handling scenarios
• Logging and diagnostic use cases that preserve debugging information

Technical Benefits Explained

The documentation now clearly explains why this approach is superior:

try
{
    ProcessFile();
}
catch (IOException ex) when (ex.Message.Contains("access denied"))
{
    // Filter evaluated before stack unwinding
    // Full debugging context still available
    HandleAccessDenied();
}
catch (IOException ex)
{
    // Traditional approach - stack already unwound
    if (ex.Message.Contains("not found"))
    {
        HandleNotFound();
    }
}

The exception filter approach is particularly valuable in production applications where preserving debugging information is crucial for diagnosing issues.

Fixes #40661.

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Internal previews

📄 File	🔗 Preview link
docs/csharp/language-reference/keywords/when.md	docs/csharp/language-reference/keywords/when
docs/csharp/language-reference/statements/exception-handling-statements.md	docs/csharp/language-reference/statements/exception-handling-statements

[BillWagner]

This is a good start.

[gewarren]

Code comments should end in punctuation.

[gewarren]

"Before" is italicized in both bullet points to add emphasis, which doesn't make much sense since they're trying to point out differences.

[gewarren]

I know Copilot likes to add bolding using **, but it actually goes against style guidelines.

[gewarren]

This shouldn't be a numbered list since it's not a sequential procedure.

[gewarren]

Add periods.