Add C# 9 diagnostics for record types

[BillWagner]

Fixes #52834

Add the previously undocumented diagnostics for the C# 9 record type feature.

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Internal previews

📄 File	🔗 Preview link
docs/csharp/language-reference/compiler-messages/record-declaration-errors.md	Resolve errors and warnings for record declarations
docs/csharp/language-reference/toc.yml	docs/csharp/language-reference/toc
docs/csharp/misc/sorry-we-don-t-have-specifics-on-this-csharp-error.md	Coming in C# 15

[Copilot]

Pull request overview

Adds documentation coverage for previously undocumented C# 9 compiler diagnostics related to record types, and wires those diagnostics into navigation and keyword indexing so they no longer appear on the “no specifics” page.

Changes:

• Removes record-related diagnostics from the catch-all “Sorry, we don't have specifics…” list.
• Adds CS8857, CS8869, CS8871, CS8876, and CS8906 to the dedicated “record declarations” compiler-messages article (keywords, summaries, and guidance).
• Updates the C# language reference TOC search/display metadata to include the newly documented diagnostic IDs.

Reviewed changes

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

File	Description
docs/csharp/misc/sorry-we-don-t-have-specifics-on-this-csharp-error.md	Removes diagnostics that are now documented elsewhere.
docs/csharp/language-reference/toc.yml	Updates TOC metadata so the record diagnostics are discoverable via the record declarations page.
docs/csharp/language-reference/compiler-messages/record-declaration-errors.md	Documents the newly covered record diagnostics and updates keyword metadata accordingly.