Rollup merge of #63488 - RalfJung:diagnostic-docs, r=zackmdavis
Rollup merge of #63488 - RalfJung:diagnostic-docs, r=zackmdavis

improve DiagnosticBuilder docs

Cc @estebank @oli-obk

Is there any way to do something like `span_note` but with a label attached to the span? I thought `.span_note().span_label()` would do it, but no, that does not work.

graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust