Rollup merge of #146653 - jdonszelmann:empty-attr-diags, r=nnethercote

improve diagnostics for empty attributes

Adds a note about them not having any effect. This was previously done for `feature` attributes but no other attributes. In [converting the `feature` parser](rust-lang/rust#146652) I removed that note. This PR adds it back in and makes it so all attributes benefit from it.

Not blocked on rust-lang/rust#146652, either can merge first

Author: Zalathar