add-interactive: Clarify “remaining hunks in the file”

The "a" and "d" commands to ‘add --patch’ (accept/reject rest of file)
interact with "j", "g", and "/" (skip some hunks) in a perhaps
confusing way: after accepting or rejecting all _later_ hunks in the
file, they return to the earlier, skipped hunks and prompt the user
about them again.

This behavior can be very useful in practice.  One can still accept or
reject _all_ undecided hunks in a file by using the "g" command to
move to hunk #1 first.

Reported-by: Frédéric Brière <fbriere@fbriere.net>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>