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](https://github.com/digint/btrbk/issues/190). 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.

Start by reading the manual entries for snapshot_preserve_min, target_preserv_min, snapshot_preserve, and target_preserve, then verify how retention handles manual snapshots and the value all. Done means documenting whether count-based retention exists, how the option pairs interact, and why setting the _min options to all affects the other options.

Written by the indexing model from the issue text.