Document use of Cdecl in binding to varargs.

This is the first binding to a varargs function that I could find in the
library, so make it clear why this calling convention was used.

Author: Ryan Roden-Corrent

LibGit2Sharp/Core/NativeMethods.cs

@@ -584,10 +584,10 @@ internal static extern int git_filter_unregister(
         #region git_libgit2_opts
 
         // Bindings for git_libgit2_opts(int option, ...):
-        // Varargs can be bound using __arglist in .NET, but I had trouble getting that working with Mono.
-        // Instead of using __arglist, I enumerate the possible signatures here.
         // Currently only GIT_OPT_GET_SEARCH_PATH and GIT_OPT_SET_SEARCH_PATH are supported,
         // but other overloads could be added using a similar pattern.
+        // CallingConvention.Cdecl is used to allow binding the the C varargs signature, and each possible call signature must be enumerated.
+        // __argslist was an option, but is an undocumented feature that should likely not be used here.
 
         // git_libgit2_opts(GIT_OPT_GET_SEARCH_PATH, int level, git_buf *buf)
         [DllImport(libgit2, CallingConvention = CallingConvention.Cdecl)]