Rollup merge of #121689 - GuillaumeGomez:rustdoc-highlighting-whitespace, r=notriddle

[rustdoc] Prevent inclusion of whitespace character after macro_rules ident

Discovered this bug randomly when looking at:

![image](https://github.com/rust-lang/rust/assets/3050060/dca38047-9085-4377-bfac-f98890224be4)

We were too eagerly trying to merge tokens that shouldn't be merged together (for example if you have a code comment followed by a code comment, we merge them in one attribute to reduce the DOM size).

r? ``@notriddle``

Author: matthiaskrgr