doc: notes: treat --stdin equally between copy/remove

doc: notes: treat --stdin equally between copy/remove

46538012d94 (notes remove: --stdin reads from the standard input,
2011-05-18) added `--stdin` for the `remove` subcommand, documenting it
in the “Options” section.  But `copy --stdin` was added before that, in
160baa0d9cb (notes: implement 'git notes copy --stdin', 2010-03-12).

Treat this option equally between the two subcommands:

• remove: mention `--stdin` on the subcommand as well, like for `copy`
• copy: mention it as well under the option documentation

Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name>
Signed-off-by: Junio C Hamano <gitster@pobox.com>