Rollup merge of #145356 - GuillaumeGomez:doctest-escaped, r=fmease

Add another example for escaped `#` character in doctest in rustdoc book

Fixes rust-lang/rust#118027.

We don't plan to add a way to not escape the first `#`, so at least we add a more complete example in the rustdoc book with a macro making use of that.

Author: GuillaumeGomez