Revert "Cleanup markdown span handling"
Reverts rust-lang/rust#80244. This caused a diagnostic regression, originally it was:
```
warning: unresolved link to `std::process::Comman`
--> link.rs:3:10
|
3 | //! [a]: std::process::Comman
| ^^^^^^^^^^^^^^^^^^^^ no item named `Comman` in module `process`
|
= note: `#[warn(broken_intra_doc_links)]` on by default
```
but after that PR rustdoc now displays
```
warning: unresolved link to `std::process::Comman`
--> link.rs:1:14
|
1 | //! Links to [a] [link][a]
| ^^^ no item named `Comman` in module `process`
|
= note: `#[warn(broken_intra_doc_links)]` on by default
```
which IMO is much less clear.
cc `@bugadani,` thanks for catching this in https://github.com/rust-lang/rust/pull/77859.
r? `@GuillaumeGomez`
7494aef979