Revert "Cleanup markdown span handling"

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.

1 files changed, 7 insertions, 0 deletions

diff --git a/src/test/rustdoc-ui/reference-links.rs b/src/test/rustdoc-ui/reference-links.rs
new file mode 100644
index 00000000000..7c1a79722c9
--- /dev/null
+++ b/src/test/rustdoc-ui/reference-links.rs
@@ -0,0 +1,7 @@
+// Test that errors point to the reference, not to the title text.
+#![deny(broken_intra_doc_links)]
+//! Links to [a] [link][a]
+//!
+//! [a]: std::process::Comman
+//~^ ERROR unresolved
+//~| ERROR unresolved