Make `prune_interval` more clear to users

[robinbb]

'prune_interval' is the multiple of the height of the transition frontier at which tree pruning occurs. The default is 10. Perhaps this is too high? Or maybe no matter? Why prune the tree at all (what traverses it)? At the least, the documentation for users about how they should set this is lacking. Maybe it should be called "tree-height-prune-limit"? ...