Fix clippy complaints

[Vrixyz]

Fixes a few clippy warnings.

doc(cfg(..)

Currently, https://docs.rs/nalgebra/latest/nalgebra/base/par_iter/struct.ParColumnIter.html doesn't warn that the feature rayon should be activated, well it's written in comments, but it should be covered by the line:

#[cfg_attr(doc_cfg, doc(cfg(feature = "rayon")))]

But it is not, because doc_cfg is a feature, so it should be written: #[cfg_attr(feature = "doc_cfg", doc(cfg(feature = "rayon")))].

BUT now rust requires all features to be listed in Cargo.toml, or it will complain about "unexpected cfg", I think it makes little sense to go through an intermediate feature, which would add noise to existing feature list, so I just replaced it by docsrs compile flag, which works as showcased:

Note

• To be noted, Implement RFC 3631: add rustdoc doc_cfg features rust-lang/rust#138907 may make this PR a bit obsolete, where we may get rid of those markers, but this PR is an improvement in the meantime.