Improve RootSet documentation

RootSet.h

@@ -15,7 +15,8 @@ struct RootSet {
 
   /// Foreign threads may freely interact with a `ThreadSafeHandle`, as long as
   /// the latest version as of obtaining the handle is greater than
-  /// `oldestVersion`.
+  /// `oldestVersion`, and they only read versions >= `oldestVersion` and <= the
+  /// version as of obtaining the handle.
   struct ThreadSafeHandle {
 
     /// Get a root node that can correctly be used for `version`