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>

Documentation/git-notes.txt

@@ -142,8 +142,9 @@ OPTIONS
 
 -C <object>::
 --reuse-message=<object>::
-	Take the note message from the given blob object (for
-	example, another note).
+	Take the given blob object (for	example, another note) as the
+	note message. (Use `git notes copy <object>` instead to
+	copy notes between objects.)
 
 -c <object>::
 --reedit-message=<object>::
@@ -285,6 +286,8 @@ $ blob=$(git hash-object -w a.out)
 $ git notes --ref=built add -C "$blob" HEAD
 ------------
 
+(You cannot simply use `git notes --ref=built add -F a.out HEAD`
+because that is not binary-safe.)
 Of course, it doesn't make much sense to display non-text-format notes
 with 'git log', so if you use such notes, you'll probably need to write
 some special-purpose tools to do something useful with them.