[5.x] Fix misleading memory limit config

[paras-malhotra]

The memory_limit Horizon configuration is misleading. It reads that it's the max memory for the queue workers, when instead, it is the memory limit for the master supervisor (default 64 MB). The memory limit for the workers percolates down from the supervisor memory limit (default 128 MB).

This PR fixes the wording and also makes it clear that the supervisor memory option is the one for the queue workers. This is a non-breaking change, as the config defaults are the same (64 MB for master supervisor and 128 MB for supervisors + queue workers).

Edit: On digging further, I realised this issue has been lingering from quite some time now: #712