Clarify documentation regarding packsize defaults

[simonsan]

I think this is more a documentation/clarification issue. Rustic has a dynamic targeted packsize depending on the repository size.
This means, if you increase the repository size, rustic does create large pack files and prune will try to repack packs with non-matching sizes. The algorithm about determining the target pack size and the thresholds when rustic will try to repack too small or too large sized packs are saved in the repository config file and can be set using rustic config (add --help to see the possible options).
However, I'm not sure if the chosen defaults in the for the tuning parameters can be improved, so feedback about it is highly welcome!

The idea about pack sizes depending on the repository size is that you get a good fit for individual repsoitories without the need to set/guess something optimal. In general, rustic's pack sizes are larger than restics and can go up to almost 4GiB.
The idea about repacking too small / or too large packs is that for a stable situation (periodic backups, periodic forget/prune), your repository size should stay almost constant, while this usually only changes at one-time events like adding new sources etc. And then a one-time prune to correct some pack sizes IMO is OK.

To ignore too small or too large packs, there is prune --no-resize which should behave exactly like restic.

Originally posted by @aawsome in rustic-rs/rustic#1017 (comment)