man: Add man page for create-usb

Closes: #1543
Approved by: cgwalters

src/libostree/ostree-repo-finder-mount.c

@@ -434,7 +434,8 @@ G_GNUC_END_IGNORE_DEPRECATIONS
       g_array_sort (repos_refs, repo_and_refs_compare);
 
       /* Also check the well-known special-case directories in the mount.
-       * Add them after sorting, so they’re always last. */
+       * Add them after sorting, so they’re always last.
+       * NOTE: If you change these, update the man page. */
       const gchar * const well_known_repos[] =
         {
           ".ostree/repo",

src/ostree/ot-builtin-create-usb.c

@@ -41,7 +41,6 @@ static GOptionEntry options[] =
     { NULL }
   };
 
-/* TODO: Add a man page. */
 gboolean
 ostree_builtin_create_usb (int            argc,
                            char         **argv,