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/commit.c

@@ -1472,7 +1472,7 @@ int cmd_commit(int argc, const char **argv, const char *prefix)
 		OPT_BOOL('e', "edit", &edit_flag, N_("force edit of commit")),
 		OPT_STRING(0, "cleanup", &cleanup_arg, N_("default"), N_("how to strip spaces and #comments from message")),
 		OPT_BOOL(0, "status", &include_status, N_("include status in commit message template")),
-		{ OPTION_STRING, 'S', "gpg-sign", &sign_commit, N_("key id"),
+		{ OPTION_STRING, 'S', "gpg-sign", &sign_commit, N_("key-id"),
 		  N_("GPG sign commit"), PARSE_OPT_OPTARG, NULL, (intptr_t) "" },
 		/* end commit message options */