Define meaning of BuckStopsHereBinder.AssociatedSyntaxTree in EE scenarios

[RikkiGibson]

Related to #60819

roslyn/src/Compilers/CSharp/Portable/Binder/BuckStopsHereBinder.cs

Lines 24 to 31 in a540733

	/// <summary>
	/// In non-speculative scenarios, the syntax tree being bound.
	/// In speculative scenarios, the syntax tree from the original compilation used as the speculation context.
	/// This is <see langword="null"/> in some scenarios, such as the binder used for <see cref="CSharpCompilation.Conversions" />
	/// or the binder used to bind usings in <see cref="CSharpCompilation.UsingsFromOptionsAndDiagnostics"/>.
	/// PROTOTYPE(ft): what about in EE scenarios?
	/// </summary>
	internal readonly SyntaxTree? AssociatedSyntaxTree;