parse-options: multi-word argh should use dash to separate words

"When you need to use space, use dash" is a strange way to say that
you must not use a space.  Because it is more common for the command
line descriptions to use dashed-multi-words, you do not even want to
use spaces in these places.  Rephrase the documentation to avoid
this strangeness.

Fix a few existing multi-word argument help strings, i.e.

 - GPG key-ids given to -S/--gpg-sign are "key-id";
 - Refs used for storing notes are "notes-ref"; and
 - Expiry timestamps given to --expire are "expiry-date".

and update the corresponding documentation pages.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/notes.c

@@ -939,7 +939,7 @@ int cmd_notes(int argc, const char **argv, const char *prefix)
 	int result;
 	const char *override_notes_ref = NULL;
 	struct option options[] = {
-		OPT_STRING(0, "ref", &override_notes_ref, N_("notes_ref"),
+		OPT_STRING(0, "ref", &override_notes_ref, N_("notes-ref"),
 			   N_("use notes from <notes_ref>")),
 		OPT_END()
 	};