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/merge.c

@@ -220,7 +220,7 @@ static struct option builtin_merge_options[] = {
 	OPT_BOOL(0, "abort", &abort_current_merge,
 		N_("abort the current in-progress merge")),
 	OPT_SET_INT(0, "progress", &show_progress, N_("force progress reporting"), 1),
-	{ 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) "" },
 	OPT_BOOL(0, "overwrite-ignore", &overwrite_ignore, N_("update ignored files (default)")),
 	OPT_END()