Improve documentation of 0100664 mode usage

Author: nulltoken

LibGit2Sharp/Mode.cs

@@ -23,7 +23,12 @@ public enum Mode
         NonExecutableFile = 0x81A4,
 
         /// <summary>
-        ///   100664 file mode
+        ///   Obsolete 100664 file mode.
+        ///   <para>0100664 mode is an early Git design mistake. It's kept for
+        ///     ascendant compatibility as some <see cref="Tree"/> and
+        ///     <see cref="Repository.Index"/> entries may still bear
+	    ///     this mode in some old git repositories, but it's now deprecated.
+        ///   </para>
         /// </summary>
         NonExecutableGroupWritableFile = 0x81B4,