improve some comments

Author: huynhsontung

Microsoft.Toolkit.Uwp.UI.Controls.Input/RichSuggestBox/RichSuggestBox.Properties.cs

@@ -313,6 +313,9 @@ public SolidColorBrush TokenForeground
         /// <summary>
         /// Gets or sets prefix characters to start a query.
         /// </summary>
+        /// <remarks>
+        /// Prefix characters must be punctuations (must satisfy <see cref="char.IsPunctuation(char)"/> method).
+        /// </remarks>
         public string Prefixes
         {
             get => (string)GetValue(PrefixesProperty);

Microsoft.Toolkit.Uwp.UI.Controls.Input/RichSuggestBox/RichSuggestBox.cs

@@ -75,8 +75,7 @@ public RichSuggestBox()
         }
 
         /// <summary>
-        /// Clear unused tokens and undo/redo history. <see cref="RichSuggestBox"/> saves all of previously committed tokens
-        /// even when they are removed from the text. They have to be manually removed using this method.
+        /// Clear unused tokens and undo/redo history.
         /// </summary>
         public void ClearUndoRedoSuggestionHistory()
         {