Add triple slash comments for new System.Drawing API added in 6.0 (#57313)

* Add triple slash comments for new System.Drawing API added in 6.0

Co-authored-by: Genevieve Warren <24882762+gewarren@users.noreply.github.com>

Author: safern

src/libraries/Common/src/System/Drawing/KnownColor.cs

@@ -197,6 +197,9 @@ public enum KnownColor
         MenuHighlight,
 
         // "Web" colors, Part 2
+        /// <summary>
+        /// A system defined color representing the ARGB value <c>#663399</c>.
+        /// </summary>
         RebeccaPurple,
     }
 }

src/libraries/System.Drawing.Primitives/src/System/Drawing/Color.cs

@@ -249,6 +249,10 @@ namespace System.Drawing
 
         public static Color Purple => new Color(KnownColor.Purple);
 
+        /// <summary>
+        /// Gets a system-defined color that has an ARGB value of <c>#663399</c>.
+        /// </summary>
+        /// <value>A system-defined color.</value>
         public static Color RebeccaPurple => new Color(KnownColor.RebeccaPurple);
 
         public static Color Red => new Color(KnownColor.Red);