git-notes.txt: clarify -C vs. copy and -F

git-notes.txt: clarify -C vs. copy and -F

The current description of '-C' together with the analogy to 'git commit
-C' can lead to the wrong conclusion that '-C' copies notes between
objects. Make this clearer by rewording and pointing to 'copy'.

The example for attaching binary notes with 'git hash-object' followed
by 'git notes add -C' immediately raises the question: "Why not use 'git
notes add -F'?". Answer it (the latter is not binary-safe).

Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Acked-by: Johan Herland <johan@herland.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>