Add a comment to better explain GoalState.IsAchieved

src/LibraryManager.Contracts/LibraryInstallationGoalState.cs

@@ -33,6 +33,11 @@ public LibraryInstallationGoalState(ILibraryInstallationState installationState,
         /// <summary>
         /// Returns whether the goal is in an achieved state - that is, all files are up to date.
         /// </summary>
+        /// <remarks>
+        /// This is intended to serve as a fast check compared to restoring the files.  
+        /// If there isn't a faster way to verify that a file is up to date, this method should
+        /// return false to indicate that a restore can't be skipped.
+        /// </remarks>
         public bool IsAchieved()
         {
             foreach (KeyValuePair<string, string> kvp in InstalledFiles)