Rollup merge of #75389 - RalfJung:span_label, r=davidtwco

attempt to improve span_label docs

I was still confused by the `span_label` docs, so I did some more digging. However, this needs careful checking as I have no idea if any of this is correct.

diff --cc src/librustc_errors/diagnostic.rs
Simple merge

diff --cc src/librustc_errors/diagnostic_builder.rs
Simple merge