Improve info message for acyclic models

[Maximilian-Stefan-Ernst]

Atm, specifying an acyclic model but no uper/lower triangular A Matrix says: "Your model is acyclic, specifying the A Matrix as either Upper or Lower Triangular can have great performance benefits.\n". I believe it should also refer to a section of the docs where model sorting is explained, because right now it might be quite confusing for users not familiar with this.