Clearification of return value.

I wasn't sure what happens if no repo is found. (does it return null or throws  an exception).
So I thought it could be stated clearer for people like me.

Author: LokiMidgard

LibGit2Sharp/Repository.cs

@@ -604,7 +604,7 @@ public static IEnumerable<Reference> ListRemoteReferences(string url, Credential
         /// <para>The lookup start from <paramref name="startingPath"/> and walk upward parent directories if nothing has been found.</para>
         /// </summary>
         /// <param name="startingPath">The base path where the lookup starts.</param>
-        /// <returns>The path to the git repository.</returns>
+        /// <returns>The path to the git repository, or null if no repository was found.</returns>
         public static string Discover(string startingPath)
         {
             FilePath discoveredPath = Proxy.git_repository_discover(startingPath);