clarify `-a` option for `beet modify`
clarify `-a` option for `beet modify`

See https://github.com/beetbox/beets/discussions/4172.

I think the confusion arises because the documentation refers to the
query. That is, when `-a` is given, albums are queried, not tracks. This
is especially clear when using `beet list`, because then it truly lists
"albums instead of items". However, for other commands, the distinction
between what is queried and what is acted on should be made more clear.

This PR fixes the section for `modify`, but there are more questions:

- `remove` command: The documentation states that it acts on albums
instead of individual tracks. I guess we should also amend that? I think
the complete album including the tracks is deleted, or is that not true?

- `move` command: I think the same is true for this command. If `-a` is
  given, the queried albums including all tracks are moved.

- `update` command: The `-a` flag is not explained here.

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