Add XML comments to public entities.

Aaru.Core/Error.cs

@@ -34,8 +34,16 @@ using Aaru.CommonTypes.Interop;
 
 namespace Aaru.Core
 {
+    /// <summary>
+    /// Prints the description of a system error number.
+    /// </summary>
     public static class Error
     {
+        /// <summary>
+        /// Prints the description of a system error number.
+        /// </summary>
+        /// <param name="errno">System error number.</param>
+        /// <returns>Error description.</returns>
         public static string Print(int errno)
         {
             switch(DetectOS.GetRealPlatformID())