Extension methods section isn't complete

Extension methods section doesn't show when the multiple extension methods with the **same name** exists in **different namespaces**.

For example:

It only shows "IsKind(SyntaxToken, SyntaxKind)" from , but doesn't show .

Start by comparing the SyntaxToken API page with the linked CSharpExtensions.IsKind and VisualBasicExtensions.IsKind entries. Trace how the extension-method documentation is assembled for methods sharing a name across namespaces. Done means both IsKind overloads appear in the SyntaxToken extension methods section.

索引モデルが issue の本文から書いたものです。