Add watermark property comments

src/Avalonia.Controls/TextBox.cs

@@ -426,16 +426,24 @@ namespace Avalonia.Controls
             set { SetValue(TextAlignmentProperty, value); }
         }
 
+        /// <summary>
+        /// Gets or sets the placeholder or descriptive text that is displayed even if the <see cref="Text"/>
+        /// property is not yet set.
+        /// </summary>
         public string? Watermark
         {
-            get { return GetValue(WatermarkProperty); }
-            set { SetValue(WatermarkProperty, value); }
+            get => GetValue(WatermarkProperty);
+            set => SetValue(WatermarkProperty, value);
         }
 
+        /// <summary>
+        /// Gets or sets a value indicating whether the <see cref="Watermark"/> will still be shown above the
+        /// <see cref="Text"/> even after a text value is set.
+        /// </summary>
         public bool UseFloatingWatermark
         {
-            get { return GetValue(UseFloatingWatermarkProperty); }
-            set { SetValue(UseFloatingWatermarkProperty, value); }
+            get => GetValue(UseFloatingWatermarkProperty);
+            set => SetValue(UseFloatingWatermarkProperty, value);
         }
 
         public object InnerLeftContent