MEMORY DOCTOR 命令会列出 Redis 服务器遇到的内存相关问题，并提供相应的解决建议。
*返回值
*例子
127.0.0.1:5379> MEMORY DOCTOR
Hi Sam, this instance is empty or is using very little memory, my issues detector can't be used in these conditions. Please, leave for your mission on Earth and fill it with some data. The new Sam and I will be back to our programming as soon as I finished rebooting.
127.0.0.1:5379>