Various manpage / usage blurb improvements

Corrections, clarifications, consistency.

Not a comprehensive overhaul of the manpages; they do still need work.
These are just flaws I've noted while studying the ostree command syntax.

https://bugzilla.gnome.org/show_bug.cgi?id=740097

src/ostree/ot-builtin-cat.c

@@ -65,7 +65,7 @@ ostree_builtin_cat (int argc, char **argv, OstreeRepo *repo, GCancellable *cance
   gs_unref_object GFile *root = NULL;
   gs_unref_object GFile *f = NULL;
 
-  context = g_option_context_new ("COMMIT PATH [PATH...] - Concatenate contents of files");
+  context = g_option_context_new ("COMMIT PATH... - Concatenate contents of files");
   g_option_context_add_main_entries (context, options, NULL);
 
   if (!g_option_context_parse (context, &argc, &argv, error))