Documentation/git-remote-helpers: explain how import works with multiple refs

Documentation/git-remote-helpers: explain how import works with multiple refs

This is important for two reasons:

* when two "import" lines follow each other, only one "done" command
  should be issued in the fast-import stream, not one per "import".

* The blank line terminating an import command should not be confused
  with the one terminating the sequence of commands.

While we're there, illustrate the corresponding explanation for push
batches with an example.

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Acked-by: Sverre Rabbelier <srabbelier@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>