Improve Network.ListReferences() documentation

Author: nulltoken

LibGit2Sharp/Network.cs

@@ -54,6 +54,12 @@ public virtual RemoteCollection Remotes
 
         /// <summary>
         /// List references in a <see cref="Remote"/> repository.
+        /// <para>
+        /// When the remote tips are ahead of the local ones, the retrieved
+        /// <see cref="DirectReference"/>s may point to non  existing
+        /// <see cref="GitObject"/>s in the local repository. In that
+        /// case, <see cref="DirectReference.Target"/> will return <c>null</c>.
+        /// </para>
         /// </summary>
         /// <param name="remote">The <see cref="Remote"/> to list from.</param>
         /// <returns>The references in the <see cref="Remote"/> repository.</returns>