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-checksum.c

@@ -63,7 +63,7 @@ ostree_builtin_checksum (int argc, char **argv, OstreeRepo *repo, GCancellable *
   gs_unref_object GFile *f = NULL;
   AsyncChecksumData data = { 0, };
 
-  context = g_option_context_new ("FILENAME - Checksum a file or directory");
+  context = g_option_context_new ("PATH - Checksum a file or directory");
   g_option_context_add_main_entries (context, options, NULL);
 
   if (!g_option_context_parse (context, &argc, &argv, error))