rebase -i: remind that the lines are top-to-bottom

Nelson Benitez Leon opened a discussion with a patch with this in the
note:

    Hi, I was using git rebase -i for some time now and never occured to
    me I could reorder the commit lines to affect the order the commits
    are applied, learnt that recently from a git tutorial.

Nelson's patch was to stress the fact that the lines in the insn sheet can
be re-ordered in a much more verbose way.  Let's add a one-liner reminder
and also remind that the lines in the insn sheet is read from top to
bottom, unlike the "git log" output.

Discussion-triggered-by: Nelson Benitez Leon
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-rebase--interactive.sh

@@ -846,6 +846,8 @@ cat >> "$todo" << EOF
 #  f, fixup = like "squash", but discard this commit's log message
 #  x, exec = run command (the rest of the line) using shell
 #
+# These lines can be re-ordered; they are executed from top to bottom.
+#
 # If you remove a line here THAT COMMIT WILL BE LOST.
 # However, if you remove everything, the rebase will be aborted.
 #