Don't disallow adding a DIM to an interface

[gewarren]

Fixes #42748.
Also reorganizes interface keyword doc as it didn't seem to flow properly.

cc @jskeet.

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Internal previews

📄 File	🔗 Preview link
docs/core/compatibility/library-change-rules.md	Change rules for compatibility
docs/csharp/language-reference/keywords/interface.md	:::no-loc text="interface"::: (C# Reference)
docs/csharp/whats-new/csharp-version-history.md	The history of C#

[jskeet]

Thanks so much for this!

[BillWagner]

Thanks @gewarren

This looks good, with a few small changes.

[Copilot]

Pull Request Overview

This PR updates documentation and rules regarding default interface members (DIMs) to clarify that adding a DIM to an interface is not strictly disallowed, but requires judgment. The changes correct terminology and reorganize content for better flow.

• Updates library change rules to allow adding static, non-abstract, non-virtual members to interfaces in certain scenarios
• Corrects terminology from "default interface methods" to "default interface members" for consistency
• Reorganizes the interface keyword documentation to improve readability and flow

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
docs/core/compatibility/library-change-rules.md	Updates breaking change rules to allow adding DIMs to interfaces with judgment, removes extra whitespace
docs/csharp/language-reference/keywords/interface.md	Reorganizes content structure and corrects access modifier descriptions for better clarity
docs/csharp/whats-new/csharp-version-history.md	Corrects terminology from "methods" to "members" and fixes comma placement