rustdoc: Fix doctest heuristic for main fn wrapping
Fixes#140412 which regressed in #140220 that I reviewed. As mentioned in https://github.com/rust-lang/rust/pull/140220#issuecomment-2837061779, at the time I didn't have the time to re-review its latest changes and should've therefore invalided my previous "r=me" and blocked the PR on another review given the fragile nature of the doctest impl. This didn't happen which is my fault.
Contains some other small changes. Diff best reviewed modulo whitespace.
r? ``@GuillaumeGomez``
96faee497a