Redis number of clients has to be the same or higher than the amount of nodes

https://github.com/redis/redis/blob/56a802057e90ea482702dbd5191bc397caf0f596/src/redis-benchmark.c#L684

It should be corrected or documented that the number of clients has to be the same or higher than the amount of nodes.

Comment From: yossigo

@DanEidelman Do you want to open a PR for that?