Rollup merge of #28761 - steveklabnik:gh28082, r=brson
Rollup merge of #28761 - steveklabnik:gh28082, r=brson

We don't completely cover documentation tests in the testing chapter,
since we cover them in the documentation chapter. So make sure people
know that.

Fixes #28082

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