Added XML docs for StringFormatValueConverter.

src/Avalonia.Base/Data/Converters/StringFormatValueConverter.cs

@@ -3,8 +3,18 @@ using System.Globalization;
 
 namespace Avalonia.Data.Converters
 {
+    /// <summary>
+    /// A value converter which calls <see cref="string.Format(string, object)"/>
+    /// </summary>
     public class StringFormatValueConverter : IValueConverter
     {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="StringFormatValueConverter"/> class.
+        /// </summary>
+        /// <param name="format">The format string.</param>
+        /// <param name="inner">
+        /// An optional inner converter to be called before the format takes place.
+        /// </param>
         public StringFormatValueConverter(string format, IValueConverter inner)
         {
             Contract.Requires<ArgumentNullException>(format != null);
@@ -13,15 +23,24 @@ namespace Avalonia.Data.Converters
             Inner = inner;
         }
 
+        /// <summary>
+        /// Gets an inner value converter which will be called before the string format takes place.
+        /// </summary>
         public IValueConverter Inner { get; }
+
+        /// <summary>
+        /// Gets the format string.
+        /// </summary>
         public string Format { get; }
 
+        /// <inheritdoc/>
         public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
         {
             value = Inner?.Convert(value, targetType, parameter, culture) ?? value;
             return string.Format(Format, value, culture);
         }
 
+        /// <inheritdoc/>
         public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
         {
             throw new NotSupportedException("Two way bindings are not supported with a string format");