do not vertically align list items in case the tactic is Horizontal

[scampi]

part of #2633

This does not close the issue because the use case described in #2633 (comment) is not fixed, but the rest seems fine.

[scampi]

The integration test of stdsimd fails with an issue similar to #3008.

 /// ```rust
-/// # #![cfg_attr(not(dox), feature(cfg_target_feature, target_feature, stdsimd))]
+/// # #![cfg_attr(not(dox), feature(cfg_target_feature, target_feature,
+/// stdsimd))]
 /// ...
 /// ```

• a first run of rustfmt reformats the attribute, leaving stdsimd on a new line but uncommented.
• rustfmt tries to format that code block again but fails and exit with 101 because of that new line.

[nrc]

Thanks!