Add doc

src/Workspaces/Core/Portable/FindSymbols/SymbolFinder_Hierarchy_Legacy.cs

@@ -17,6 +17,11 @@
 
 namespace Microsoft.CodeAnalysis.FindSymbols
 {
+    // This file contains the legacy SymbolFinder APIs.  The APIs are legacy because they
+    // do not contain enough information for us to effectively remote them over to the OOP
+    // process to do the work.  Specifically, they lack the "current project context" necessary
+    // to be able to effectively serialize symbols to/from the remote process.
+
     public static partial class SymbolFinder
     {
         /// <summary>