Comment syntax fixes

src/Avalonia.Base/AvaloniaObjectExtensions.cs

@@ -334,7 +334,7 @@ namespace Avalonia
         /// <typeparamref name="TTarget"/>.
         /// </summary>
         /// <typeparam name="TTarget">The type of the property change sender.</typeparam>
-        /// /// <typeparam name="TValue">The type of the property.</typeparam>
+        /// <typeparam name="TValue">The type of the property.</typeparam>
         /// <param name="observable">The property changed observable.</param>
         /// <param name="action">
         /// The method to call. The parameters are the sender and the event args.

src/Avalonia.Base/Media/TextFormatting/Unicode/BiDiAlgorithm.cs

@@ -687,7 +687,7 @@ namespace Avalonia.Media.TextFormatting.Unicode
         /// <remarks>
         /// This method resolves the sos and eos values for the run
         /// and adds the run to the list
-        /// /// </remarks>
+        /// </remarks>
         /// <param name="start">The index of the start of the run (in x9 removed units)</param>
         /// <param name="length">The length of the run (in x9 removed units)</param>
         /// <param name="level">The level of the run</param>

src/Avalonia.Controls/Primitives/PopupPositioning/IPopupPositioner.cs

@@ -216,7 +216,7 @@ namespace Avalonia.Controls.Primitives.PopupPositioning
         /// 
         /// If the adjusted position also ends up being constrained, the resulting position of the
         /// FlipX adjustment will be the one before the adjustment.
-        /// /// </remarks>
+        /// </remarks>
         FlipX = 4,
 
         /// <summary>