Document interaction between min and non-min options

I find the documentation of the snapshot_preserve_min and target_preserv_min options a bit confusing.

First of all, the manual for those options says "regardless of how many there are", seeming to imply that there's a way to count backups/snapshots and remove the ones in excess. This doesn't seem to be possible: all retention options are time-based. Is there a way to declare "only keep the last 10 snapshots"? If so, then the _min options would make a lot more sense.

Secondly, it's not clear why there are two sets of options in the first place. Somewhere on IRC it was mentioned that the _min options are there to make sure that snapshots/backups produced during manual runs are preserved: how does this work in practice? Why would they be removed without the _min options? Presumably they would follow the same filename naming scheme, so they could be handled by snapshot_preserve/target_preserve.

Then there's the issue that if snapshot_preserve_min/target_preserve_min are set to all, that snapshot_preserve/target_preserve are effectively meaningless. This should also be documented.

Please let me know if I'm right on the above points, and I'll be happy to improve the documentation accordingly.