Rollup merge of #85221 - ijackson:dbg-doc-re-tests, r=joshtriplett
Rollup merge of #85221 - ijackson:dbg-doc-re-tests, r=joshtriplett

dbg macro: Discuss use in tests, and slightly clarify

As discussed in a tangent in #82778.

I chose to use [semantic newlines](https://rhodesmill.org/brandon/2012/one-sentence-per-line/) in the source text but I don't mind reformatting it.

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