Update CS0460 documentation to include C# 8/9 constraint exceptions

[Copilot]

The CS0460 compiler error documentation was outdated and didn't reflect the constraint exceptions introduced in C# 8 and C# 9. The existing documentation stated that constraints cannot be specified directly on override and explicit interface implementation methods, but this is no longer entirely accurate.

Changes Made

• Updated description to clarify that while constraints are generally inherited, there are specific exceptions
• Added C# 9 exception for the default constraint, which can be applied to resolve ambiguities with nullable reference types
• Added C# 8 exception for explicit where T : class and where T : struct constraints to allow annotations for type parameters constrained to reference types
• Updated example code to demonstrate both the error cases (CS0460) and the valid exception cases
• Updated ms.date to reflect the documentation update

Example

The updated documentation now shows that while this still generates CS0460:

void I.F1<T>() where T : BaseClass {}   // CS0460 - cannot redeclare inherited constraint

These are now valid since C# 8 and C# 9:

// Valid since C# 8 - explicit class constraint for nullable annotations
void I.F4<T>() where T : class {}       // OK

// Valid since C# 9 - default constraint to resolve ambiguities  
void I.F3<T>() where T : default {}     // OK

Fixes #46009.

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Internal previews

📄 File	🔗 Preview link
docs/csharp/misc/cs0460.md	docs/csharp/misc/cs0460

[BillWagner]

Thanks @copilot

I have a few suggestions, and then this is ready.

[BillWagner]

Copilot made all the changes I requested in my review. This is ready for a final human review.

[Copilot]

Pull Request Overview

This PR updates the CS0460 error documentation to accurately reflect the exceptions introduced in C# 8 and C# 9 regarding constraint specifications on override and explicit interface implementation methods.

• Updated metadata (ms.date, ai-usage)
• Added sections explaining C# 8 and C# 9 exceptions with corresponding sample code
• Revised example code to differentiate between error and valid usage scenarios

[Copilot]

Just as a heads up, I was blocked by some firewall rules while working on your feedback. Expand below for details.

Warning

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I tried to connect to the following addresses, but was blocked by firewall rules:

• http://168.63.129.16:80/machine/
  • Triggering command: /usr/bin/python3 -u bin/WALinuxAgent-2.13.1.1-py3.9.egg -collect-logs (http block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to my firewall allow list