Put snapshot_preserve dependency on snapshot_preserve_min into example conf file

```
# Preserve all snapshots for a minimum period of time.
#snapshot_preserve_min 1d

# Retention policy for the source snapshots.
# Ignored if snapshot_preserve_min is not set <------------ something like this
#snapshot_preserve h d w m y
```

I wasted more time this morning that I'd care to admit because I couldn't tell that `snapshot_preserve` doesn't work unless you have `snapshot_preserve_min` set. This dependency is noted in the (Debian) man page for btrbk.conf, but I didn't realise there was one and was just looking at the man page for btrbk and the example config file.

Putting the dependency in a note in the example config file would make this requirement 'louder' - and save the next doofus like me from this issue.

Locate the example config file and compare its snapshot_preserve and snapshot_preserve_min entries with the dependency noted in the btrbk.conf man page. Add a clear note explaining that snapshot_preserve is ignored unless snapshot_preserve_min is set, then verify the example config presents this relationship accurately.

Written by the indexing model from the issue text.