git_remote_push: document that NULL refspecs allowed

Author: ethomson

include/git2/remote.h

@@ -715,8 +715,8 @@ GIT_EXTERN(int) git_remote_prune(git_remote *remote, const git_remote_callbacks
  * Peform all the steps from a push.
  *
  * @param remote the remote to push to
- * @param refspecs the refspecs to use for pushing. If none are
- * passed, the configured refspecs will be used
+ * @param refspecs the refspecs to use for pushing. If NULL or an empty
+ *                 array, the configured refspecs will be used
  * @param opts options to use for this push
  */
 GIT_EXTERN(int) git_remote_push(git_remote *remote,