docs(diff): lose incorrect claim about `diff-files --diff-filter=A`

Originally, before we had `--intent-to-add`, there was no way that `git
diff-files` could see added files: if a file did not exist in the index,
`git diff-files` would not show it because it looks only at worktree
files when there is an index entry at the same path.

We used this example in the documentation of the diff options to explain
that not every `--diff-filter=<option>` has an effect in all scenarios.

Even when we added `--intent-to-add`, the comment was still correct,
because initially we showed such files as modified instead of added.

However, when that bug was fixed in feea6946a5b (diff-files: treat
"i-t-a" files as "not-in-index", 2020-06-20), the comment in the
documentation became incorrect.

Let's just remove it.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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