Improve XML documentation clarity for IMethodSymbol.Arity property

[Copilot]

Fixes #6596

This PR improves the XML documentation for the IMethodSymbol.Arity property to provide better clarity when viewing Quick Info tooltips in IDEs.

Changes

The documentation has been updated from:

Returns the arity of this method, or the number of type parameters it takes. A non-generic method has zero arity.

To:

Returns the arity of this method. Arity is the number of type parameters a method takes. A non-generic method has zero arity.

Rationale

The new documentation provides better clarity by:

1. First stating what the property returns
2. Then providing an explicit definition of "arity" as a concept
3. Maintaining the clarification about non-generic methods

This restructuring makes the documentation more digestible and educational for developers who may not be familiar with the term "arity".

Scope

The documentation was updated consistently across all four locations where the Arity property is documented:

• IMethodSymbol.cs (public API interface)
• MethodSymbol.cs (C# compiler implementation)
• MethodSymbol.vb (Visual Basic compiler implementation)
• SynthesizedDelegateMethodSymbol.vb (Visual Basic synthesized symbols)

These changes are documentation-only and do not affect any functionality or behavior of the compiler.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Minor quick info improvement for "Arity"</issue_title>
<issue_description>

The quick info documentation for "Arity" (which is already great) can be improved slightly. My suggestion - Returns the arity of this method. Arity is the number of type parameters a method takes. A non-generic method has zero arity.
</issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes #6959

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[CyrusNajmabadi]

@dotnet/roslyn-compiler ptal.