Duplicates plugin doc does not mention explicitly the effect of `--full` on copy/move/delete

### Problem

As title says, here are the mention of the argument and the config block:

https://beets.readthedocs.io/en/stable/plugins/duplicates.html
```
-F, --full show all versions of duplicate tracks or albums
full: List every track or album that has duplicates, not just the duplicates themselves. Default: no
```

The copy/move/delete flags all mention taking effects on "matched" items, but nowhere can we read that the matched items are the one listed by the command and not the "files other than the best elected file to keep", which lead me to apply the move command to all my versions of duplicates.
Either reword copy/move/delete, or the --full to state that it "modifies the matched items to include all version of duplicate tracks/albums", which would then be consistent across the different options.

Start with the duplicates plugin documentation at https://beets.readthedocs.io/en/stable/plugins/duplicates.html and compare the --full description with the copy, move, and delete option text. Done when the documentation clearly explains which items those actions affect when --full is enabled.

Written by the indexing model from the issue text.