Document set_alg_whitelist

[jgehrcke]

set_alg_whitelist is a useful part of the interface which is so far not covered by the documentation: https://github.com/cdbattags/lua-resty-jwt/blob/f17d7c6ed45d59beb9fbf3bd5f50e89ead395b98/lib/resty/jwt.lua#L348