Add CS0677 workaround documentation using nint

[Copilot]

This PR adds documentation for a potential workaround to compiler error CS0677 by using nint as an alternative to long when declaring volatile fields.

The CS0677 error occurs when attempting to use the volatile keyword with types that cannot guarantee atomic access, such as long:

class TestClass
{
   private volatile long i;   // CS0677: a volatile field cannot be of the type 'long'
}

The documentation now includes a "Potential workarounds" section that explains how nint (native-sized integer) can be used as an alternative:

class TestClass
{
   private volatile nint i;   // This compiles successfully
}

The workaround section explains:

• How nint guarantees atomic access and works with volatile
• Platform considerations (32-bit vs 64-bit behavior)
• When this approach is most appropriate
• Cross-references to related documentation about native-sized integers
• General guidance on thread-safe programming alternatives

This addresses a gap in the documentation where developers encountering CS0677 with long had no guidance on potential solutions.

Fixes #45367.

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Internal previews

📄 File	🔗 Preview link
docs/csharp/misc/cs0677.md	docs/csharp/misc/cs0677

[BillWagner]

This is ready for final review, with my suggested changed.