Document Redis data structures

Created by: gabegorelick

As others have pointed out, Bull's data structures in Redis aren't documented. This makes working with them outside of Bull difficult.

More concerning, Bull's tests don't appear to verify the Redis data structures (please let me know if I'm missing where that happens). Thus, they could accidentally change without notice, which would break existing installations.

My understanding is Bull has not changed the Redis data structures since v3 was released, which means any mix of v3 clients, e.g. a v3.0.0 producer and v3.7.0 consumer, should be compatible. But it would be nice to have some guarantees on that front. The migration guide hasn't been updated since 2017, which either means there's been no breaking changes or everyone's forgotten about it 😄.