Document use of property import

This style is uncommon in this repo, so document that/why it's needed here.

Author: drewnoakes

src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Tree/Dependencies/DependenciesTreeProvider.cs

@@ -52,6 +52,9 @@ internal sealed partial class DependenciesTreeProvider : ProjectTreeProviderBase
     private readonly IProjectAccessor _projectAccessor;
     private readonly TaskDelayScheduler _debounce;
 
+    // NOTE we use a property import here as importing this via the constructor creates a loop between
+    // DependenciesTreeProvider and DependenciesTreeBuilder. A property import allows MEF to break that
+    // circular dependency.
     [Import]
     private DependenciesTreeBuilder TreeBuilder { get; set; } = null!;