Add reference and fundamentals for user defined compound assignment operators

[BillWagner]

Fixes #46100

• Publish the new speclet.
• Add text in "What's new in C# 14".
• Add updates in the .NET 10 What's new article.
• Update all relevant operator pages to change the language on overloading the associated compound assignment operators.

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📄 File	🔗 Preview link
docs/core/whats-new/dotnet-10/overview.md	What's new in .NET 10
docs/csharp/language-reference/operators/addition-operator.md	Addition operators - + and +=
docs/csharp/language-reference/operators/arithmetic-operators.md	Arithmetic operators (C# reference)
docs/csharp/language-reference/operators/assignment-operator.md	"Assignment operators - assign an expression to a variable"
docs/csharp/language-reference/operators/bitwise-and-shift-operators.md	Bitwise and shift operators (C# reference)
docs/csharp/language-reference/operators/boolean-logical-operators.md	docs/csharp/language-reference/operators/boolean-logical-operators
docs/csharp/language-reference/operators/operator-overloading.md	Operator overloading - predefined unary, arithmetic, equality, and comparison operators
docs/csharp/language-reference/operators/subtraction-operator.md	"- and -= operators - subtraction (minus) operators"
docs/csharp/programming-guide/classes-and-structs/access-modifiers.md	Access Modifiers (C# Programming Guide)
docs/csharp/programming-guide/classes-and-structs/members.md	docs/csharp/programming-guide/classes-and-structs/members
docs/csharp/specification/toc.yml	docs/csharp/specification/toc
docs/csharp/whats-new/csharp-14.md	What's new in C# 14

[BillWagner]

Added comments for reviewers.

[Copilot]

Pull Request Overview

This PR adds support and documentation for user-defined compound assignment, increment, and decrement operators in C# 14 across spec, language reference, and “What’s new” articles. Key changes include:

• Inclusion of a new speclet link and TOC entry for user-defined compound assignment.
• Updates to operator reference topics to describe explicit overloading rules for compound assignment, ++, and --.
• Addition of code snippets showing instance-based and void-returning compound operator implementations.

Reviewed Changes

Copilot reviewed 14 out of 14 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
docs/csharp/whats-new/csharp-14.md	Added “User defined compound assignment” section
docs/csharp/specification/toc.yml	Inserted TOC entry for user-defined compound assignment
docs/csharp/programming-guide/classes-and-structs/members.md	Modified operator member description
docs/csharp/programming-guide/classes-and-structs/access-modifiers.md	Updated access rules for user-defined operators
docs/csharp/language-reference/operators/subtraction-operator.md	Refreshed dates and overloadability text
docs/csharp/language-reference/operators/snippets/shared/OperatorOverloading.cs	Expanded struct with new compound/increment operators
docs/csharp/language-reference/operators/operator-overloading.md	Added compound assignment overload rules
docs/csharp/language-reference/operators/boolean-logical-operators.md	Updated code tags and overloadability text
docs/csharp/language-reference/operators/bitwise-and-shift-operators.md	Refreshed dates, wording, overloadability text
docs/csharp/language-reference/operators/assignment-operator.md	Refreshed dates and compound assignment note
docs/csharp/language-reference/operators/arithmetic-operators.md	Refreshed dates, wording, overloadability text
docs/csharp/language-reference/operators/addition-operator.md	Refreshed dates, wording, overloadability text
docs/core/whats-new/dotnet-10/overview.md	Added bullets for compound, increment, decrement support
docfx.json	Registered the new proposal in build configuration

Comments suppressed due to low confidence (4)

docs/csharp/language-reference/operators/snippets/shared/OperatorOverloading.cs:38

• Using operand.numerator++ returns the original value and mutates only the local copy. Replace with operand.numerator + 1 to produce the correctly incremented numerator.

public static Fraction operator ++(Fraction operand) => new Fraction(operand.numerator++, operand.denominator);

docs/csharp/language-reference/operators/snippets/shared/OperatorOverloading.cs:41

• Using operand.numerator-- returns the original value and mutates only the local copy. Replace with operand.numerator - 1 to produce the correctly decremented numerator.

public static Fraction operator --(Fraction operand) => new Fraction(operand.numerator--, operand.denominator);

docs/csharp/programming-guide/classes-and-structs/members.md:23

• Regular operator overloads must include the static modifier. Restore the wording to 'public static method' to match actual C# requirements.

|[Operators](../../language-reference/operators/index.md)|Overloaded operators are considered type members. When you overload an operator, you define it as a public method in a type. For more information, see [Operator overloading](../../language-reference/operators/operator-overloading.md).|

docs/csharp/programming-guide/classes-and-structs/access-modifiers.md:68

• The rule should require both public and static for regular operators. Update the line to reflect that user-defined operators must be public static.

User-defined operators must always be declared as `public`. For more information, see [Operator overloading](../../language-reference/operators/operator-overloading.md).