add XML docs

Author: huynhsontung

Microsoft.Toolkit.Uwp.UI/Extensions/StringExtensions.cs

@@ -182,6 +182,12 @@ public static Quaternion ToQuaternion(this string text)
             static Quaternion Throw(string text) => throw new FormatException($"Cannot convert \"{text}\" to {nameof(Quaternion)}. Use the format \"float, float, float, float\"");
         }
 
+        /// <summary>
+        /// Converts a angle bracketed <see cref="string"/> value to its unbracketed form (e.g. "&lt;float, float&gt;" to "float, float").
+        /// If the value is already unbracketed, this method will return the value unchanged.
+        /// </summary>
+        /// <param name="text">A bracketed <see cref="string"/> value.</param>
+        /// <returns>The unbracketed <see cref="string"/> value.</returns>
         private static string Unbracket(string text)
         {
             if (text.Length >= 2 &&