This is a general advice, and so shouldn't be repeated

This allows printing the labels in the same diagnostic.

Note: if we were to _only_ replaces notes by labels, but still pointed
the diagnostic at `full_span`, the resulting diagnostic would end up
rather ugly:
```
error: indexing into a slice multiple times with an `assert` that does not cover the highest index
  --> tests/ui/missing_asserts_for_indexing.rs:30:5
   |
LL |     v[0] + v[1] + v[2] + v[3] + v[4]
   |     ----^^^----^^^----^^^----^^^----
   |     |      |      |      |      |
   |     |      |      |      |      slice indexed here
   |     |      |      |      slice indexed here
   |     |      |      slice indexed here
   |     |      slice indexed here
   |     slice indexed here
   |
```