Rollup merge of #146933 - yotamofek:pr/rustdoc/highlight_no_write_str, r=GuillaumeGomez

Make `render_example_with_highlighting` return an `impl fmt::Display`

Removes some more usages of `write_str`. Shouldn't affect runtime, but makes the codebase a bit more consistent.
Each commit can be reviewed separately

Author: matthiaskrgr