Add doc

src/EditorFeatures/Core/FindUsages/IFindUsagesContext.cs

@@ -13,6 +13,10 @@ internal interface IFindUsagesContext
     {
         CancellationToken CancellationToken { get; }
 
+        /// <summary>
+        /// Used for clients that are finding usages to push information about how far along they
+        /// are in their search.
+        /// </summary>
         IStreamingProgressTracker ProgressTracker { get; }
 
         /// <summary>