Clarify delegate as built-in reference type in documentation

[Copilot]

This PR resolves confusion about whether delegate is a built-in type by clarifying the documentation in two key files.

Problem

Users were confused about whether delegate is a built-in type because:

• The built-in-types.md page included delegate in the built-in reference types table but had confusing explanatory text that said "except delegate and dynamic" when discussing aliases
• This created ambiguity about whether delegate was actually considered a built-in type

Changes Made

built-in-types.md

• Changed the confusing phrase "except delegate and dynamic" to "most C# type keywords" to eliminate the implication that delegate and dynamic aren't built-in
• Added a comprehensive explanation clarifying that delegate is indeed a built-in reference type keyword that declares types derived from the abstract System.Delegate type
• Explained that unlike other built-in type keywords, delegate isn't a direct alias but declares custom derived types

reference-types.md

• Added "built-in" qualifier to the delegate description to ensure consistency across documentation

Result

The documentation now clearly and consistently communicates that:

• delegate is a built-in reference type keyword
• It works differently from simple type aliases by enabling declaration of custom types that derive from System.Delegate
• System.Delegate itself is abstract, which is why you can't instantiate it directly
• Users create specific delegate types that inherit from this abstract base

This resolves the user's confusion while maintaining technical accuracy about how delegate types work in C#.

Fixes #28236.

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Internal previews

📄 File	🔗 Preview link
docs/csharp/language-reference/builtin-types/built-in-types.md	docs/csharp/language-reference/builtin-types/built-in-types
docs/csharp/language-reference/builtin-types/reference-types.md	docs/csharp/language-reference/builtin-types/reference-types

[BillWagner]

This LGTM and is ready for final review.