wal: sanitize wal_mode

 - add comments about modes
 - there is no need to carry NULL in wal_mode_STRS
 - use designated assignment in wal_mode_STRS

Acked-by: Serge Petrenko <sergepetrenko@tarantool.org>
Acked-by: Vladislav Shpilevoy <v.shpilevoy@tarantool.org>
Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>