Update CS0449 documentation to include all constraint types

[Copilot]

The CS0449 compiler error documentation was outdated and only mentioned 'class' and 'struct' constraints, but the current compiler error message includes additional constraint types: 'unmanaged', 'notnull', and 'default'.

This PR updates the documentation to:

1. Match current compiler error message: Updated the error description to include all 5 constraint types that can trigger CS0449
2. Clarify constraint ordering rules: Expanded explanation of how these "primary constraints" must be specified first and cannot be combined
3. Add comprehensive examples: Added 4 new example violations showing issues with the additional constraint types:
   • Combining class and notnull (invalid in nullable context since class implies notnull)
   • Combining unmanaged and struct (invalid since unmanaged implies struct)
   • Incorrect ordering of unmanaged constraint
   • Combining notnull and default constraints
4. Cross-reference documentation: Added link to the comprehensive constraints documentation page
5. Special case note: Highlighted that class already implies notnull in nullable contexts

The updated documentation now provides developers with complete information about all constraint combinations that can cause CS0449 errors, making it easier to understand and resolve these compilation issues.

Fixes #30785.

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Internal previews

📄 File	🔗 Preview link
docs/csharp/misc/cs0449.md	docs/csharp/misc/cs0449

[BillWagner]

This is ready for final review.