post-receive-email: document command-line mode

post-receive-email: document command-line mode

According to the default hooks/post-receive file, the hook is called
with three arguments on stdin:

  <oldrev> <newrev> <refname>

In command-line mode, the arguments come in a different order, because
the email hook instead calls:

  generate_email $2 $3 $1

Add a comment to explain why, based on comments from the mailing list
and the commit message to v1.5.1~9.  Thanks to Andy for the
explanation.

Requested-by: martin f. krafft <madduck@debian.org>
Cc: Andy Parkins <andyparkins@gmail.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>