add-patch: document that option J rolls over

add-patch: document that option J rolls over

The variable "permitted" is not reset after moving to a different hunk,
so it only accumulates permission and doesn't necessarily reflect those
of the current hunk.  This may be a bug, but is actually useful with the
option J, which can be used at the last hunk to roll over to the first
hunk.  Make this particular behavior official.

Also adjust the error message, as it will only be shown if there's just
a single hunk.

Suggested-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>