This PR clarifies the documentation for Encoding.GetEncoding(0) and Encoding.Default to explain their different behaviors between .NET Framework and .NET Core/later versions.

Background

In .NET Framework, both Encoding.Default and Encoding.GetEncoding(0) return the system's active code page encoding. However, in .NET Core and later versions, this behavior changed:

• Encoding.Default always returns UTF-8
• Encoding.GetEncoding(0) behavior depends on registered encoding providers

This difference can cause confusion for developers migrating code between frameworks.

Changes Made

Enhanced Encoding.Default Documentation

Added comprehensive remarks explaining that:

• In .NET Framework: Returns system's active code page (same as GetEncoding(0))
• In .NET Core+: Always returns UTF-8
• Recommends using specific Unicode encodings for cross-platform consistency

Clarified Encoding.GetEncoding(int) Behavior for codepage = 0

Updated documentation to explain platform and provider-specific behavior:

• No provider registered: Returns UTF-8 (same as Encoding.Default)
• CodePagesEncodingProvider registered: Windows returns active code page, non-Windows returns UTF-8
• Custom provider registered: Behavior determined by that provider

Updated Encoding.RegisterProvider Documentation

Enhanced explanation of how registering encoding providers affects GetEncoding(0) behavior, with specific details about CodePagesEncodingProvider's platform-specific impact.

Improved CodePagesEncodingProvider Documentation

• Added new section explaining impact on default encoding behavior
• Enhanced GetEncoding(int) method documentation with specific codepage = 0 behavior
• Replaced placeholder "To be added" remarks with comprehensive documentation

These changes help developers understand the encoding behavior differences and make informed decisions when writing cross-platform code.

Fixes #11711.

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