more comments, and fix compilation error

src/Avalonia.Visuals/Media/FontFamily.cs

@@ -53,7 +53,7 @@ namespace Avalonia.Media
         public static FontFamily Default => new FontFamily(String.Empty);
 
         /// <summary>
-        /// Represents all font families in the system
+        /// Represents all font families in the system. This can be an expensive call depending on platform implementation.
         /// </summary>
         public static IEnumerable<FontFamily> SystemFontFamilies =>
             AvaloniaLocator.Current.GetService<IPlatformRenderInterface>().InstalledFontNames.Select(name => new FontFamily(name));

tests/Avalonia.Visuals.UnitTests/VisualTree/MockRenderInterface.cs

@@ -8,6 +8,8 @@ namespace Avalonia.Visuals.UnitTests.VisualTree
 {
     class MockRenderInterface : IPlatformRenderInterface
     {
+        public IEnumerable<string> InstalledFontNames => throw new NotImplementedException();
+
         public IFormattedTextImpl CreateFormattedText(
             string text,
             Typeface typeface,