Prune help unclear about what it will delete

[gullevek]

I was a bit surpsied when my command with "--keep-daily=7" deleted (pruned) quite a lot until I realized that this meant to keep 7 days of data whatever the tags are.

Somehow this should be reflected in the documentation more clear that this will remove any data even if multiple hosts/data points are stored in the same repository and that the prefix flag should be used for per "host/data point" clean up.

Or the overall "keep-within"

[jdchristensen]

The current output of "attic help prune" is fairly long, and ends with:

If a prefix is set with -p, then only archives that start with the prefix are
considered for deletion and only those archives count towards the totals
specified by the rules.

I'm not sure about making the documentation much longer, but we could add
a sentence at the end, like

Otherwise, all archives in the repository are candidates for deletion.

What do you and Jonas think?

[ThomasWaldmann]

Maybe use prune --verbose --dry-run before doing a real prune.